wal-g doesn't seem to respect aws.credentials.ChainProvider

[kujon]

Hi 👋

I understand that there was an exactly the same issue raised a while ago and a PR that addressed it, but it seems it has been broken since moving to a separate storages package.

A simple example to reproduce would be:

# ~/.aws/credentials
[default]
aws_access_key_id = redacted
aws_secret_access_key = redacted

# ~/.aws/config
[default]
region = eu-west-1
output = json

[profile dev_admin_cli]
role_arn = arn:aws:iam::redacted:role/AdminCLI
region = eu-west-1
source_profile = default
external_id = redacted

> export AWS_PROFILE=dev_admin_cli
> aws sts get-caller-identity
{
    "Account": "redacted",
    "UserId": "redacted:botocore-session-redacted",
    "Arn": "arn:aws:sts::redacted:assumed-role/AdminCLI/botocore-session-redacted"
}

> wal-g backup-list
DEBUG: 2020/02/13 17:17:44.240330 --- COMPILED ENVIRONMENT VARS ---
DEBUG: 2020/02/13 17:17:44.241627 HOSTNAME=redacted
DEBUG: 2020/02/13 17:17:44.242767 PGPORT=5432
DEBUG: 2020/02/13 17:17:44.243837 AWS_PROFILE=dev_admin_cli
DEBUG: 2020/02/13 17:17:44.244858 PGPASSWORD=redacted
DEBUG: 2020/02/13 17:17:44.245814 AWS_REGION=eu-west-1
DEBUG: 2020/02/13 17:17:44.246788 PWD=/
DEBUG: 2020/02/13 17:17:44.247164 WALG_S3_PREFIX=s3://redacted
DEBUG: 2020/02/13 17:17:44.247274 HOME=/root
DEBUG: 2020/02/13 17:17:44.247841 PGUSER=redacted
DEBUG: 2020/02/13 17:17:44.248862 TERM=xterm
DEBUG: 2020/02/13 17:17:44.249351 WALG_LOG_LEVEL=DEVEL
DEBUG: 2020/02/13 17:17:44.249779 SHLVL=1
DEBUG: 2020/02/13 17:17:44.250353 PGHOST=redacted
DEBUG: 2020/02/13 17:17:44.250387 PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
DEBUG: 2020/02/13 17:17:44.250928 _=/usr/local/bin/wal-g
DEBUG: 2020/02/13 17:17:44.250961 WALG_DOWNLOAD_CONCURRENCY=10
DEBUG: 2020/02/13 17:17:44.251533 WALG_UPLOAD_QUEUE=2
DEBUG: 2020/02/13 17:17:44.251568 WALG_USE_WAL_DELTA=false
DEBUG: 2020/02/13 17:17:44.251690 TOTAL_BG_UPLOADED_LIMIT=32
DEBUG: 2020/02/13 17:17:44.251722 WALG_UPLOAD_CONCURRENCY=16
DEBUG: 2020/02/13 17:17:44.252356 WALG_TAR_SIZE_THRESHOLD=1073741823
DEBUG: 2020/02/13 17:17:44.252390 WALG_PREVENT_WAL_OVERWRITE=false
DEBUG: 2020/02/13 17:17:44.252511 WALG_UPLOAD_DISK_CONCURRENCY=1
DEBUG: 2020/02/13 17:17:44.253053 WALG_COMPRESSION_METHOD=lz4
DEBUG: 2020/02/13 17:17:44.253085 WALG_DELTA_MAX_STEPS=0
ERROR: 2020/02/13 17:17:49.547473 NoCredentialProviders: no valid providers in chain. Deprecated.
	For verbose messaging see aws.Config.CredentialsChainVerboseErrors
failed to get AWS credentials; please specify AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY
github.com/wal-g/wal-g/vendor/github.com/wal-g/storages/s3.createSession
	/home/travis/gopath/src/github.com/wal-g/wal-g/vendor/github.com/wal-g/storages/s3/session.go:110
github.com/wal-g/wal-g/vendor/github.com/wal-g/storages/s3.ConfigureFolder
	/home/travis/gopath/src/github.com/wal-g/wal-g/vendor/github.com/wal-g/storages/s3/folder.go:99
github.com/wal-g/wal-g/internal.ConfigureFolder
	/home/travis/gopath/src/github.com/wal-g/wal-g/internal/configure.go:124
github.com/wal-g/wal-g/cmd/pg.glob..func2
	/home/travis/gopath/src/github.com/wal-g/wal-g/cmd/pg/backup_list.go:23
github.com/wal-g/wal-g/vendor/github.com/spf13/cobra.(*Command).execute
	/home/travis/gopath/src/github.com/wal-g/wal-g/vendor/github.com/spf13/cobra/command.go:766
github.com/wal-g/wal-g/vendor/github.com/spf13/cobra.(*Command).ExecuteC
	/home/travis/gopath/src/github.com/wal-g/wal-g/vendor/github.com/spf13/cobra/command.go:852
github.com/wal-g/wal-g/vendor/github.com/spf13/cobra.(*Command).Execute
	/home/travis/gopath/src/github.com/wal-g/wal-g/vendor/github.com/spf13/cobra/command.go:800
github.com/wal-g/wal-g/cmd/pg.Execute
	/home/travis/gopath/src/github.com/wal-g/wal-g/cmd/pg/pg.go:28
main.main
	/home/travis/gopath/src/github.com/wal-g/wal-g/main/pg/main.go:8
runtime.main
	/home/travis/.gimme/versions/go1.11.13.linux.amd64/src/runtime/proc.go:201
runtime.goexit
	/home/travis/.gimme/versions/go1.11.13.linux.amd64/src/runtime/asm_amd64.s:1333
failed to create new session
github.com/wal-g/wal-g/vendor/github.com/wal-g/storages/s3.ConfigureFolder
	/home/travis/gopath/src/github.com/wal-g/wal-g/vendor/github.com/wal-g/storages/s3/folder.go:101
github.com/wal-g/wal-g/internal.ConfigureFolder
	/home/travis/gopath/src/github.com/wal-g/wal-g/internal/configure.go:124
github.com/wal-g/wal-g/cmd/pg.glob..func2
	/home/travis/gopath/src/github.com/wal-g/wal-g/cmd/pg/backup_list.go:23
github.com/wal-g/wal-g/vendor/github.com/spf13/cobra.(*Command).execute
	/home/travis/gopath/src/github.com/wal-g/wal-g/vendor/github.com/spf13/cobra/command.go:766
github.com/wal-g/wal-g/vendor/github.com/spf13/cobra.(*Command).ExecuteC
	/home/travis/gopath/src/github.com/wal-g/wal-g/vendor/github.com/spf13/cobra/command.go:852
github.com/wal-g/wal-g/vendor/github.com/spf13/cobra.(*Command).Execute
	/home/travis/gopath/src/github.com/wal-g/wal-g/vendor/github.com/spf13/cobra/command.go:800
github.com/wal-g/wal-g/cmd/pg.Execute
	/home/travis/gopath/src/github.com/wal-g/wal-g/cmd/pg/pg.go:28
main.main
	/home/travis/gopath/src/github.com/wal-g/wal-g/main/pg/main.go:8
runtime.main
	/home/travis/.gimme/versions/go1.11.13.linux.amd64/src/runtime/proc.go:201
runtime.goexit
	/home/travis/.gimme/versions/go1.11.13.linux.amd64/src/runtime/asm_amd64.s:1333

More than happy to take a shot at submitting a PR.