support .aws/config and profiles

[bortels]

Attempting to use lego with a profile or STS token fails:

2017/12/27 14:03:59 [INFO][XXXX] acme: Obtaining bundled SAN certificate
2017/12/27 14:04:00 [INFO][XXXX] AuthURL: https://acme-v01.api.letsencrypt.org/acme/authz/G4361QrN11c0XXXXXXXXXXXXX_Xa6dy_1tT9xRAbYc
2017/12/27 14:04:00 [INFO][XXXX] acme: Could not find solver for: tls-sni-01
2017/12/27 14:04:00 [INFO][XXXX] acme: Trying to solve DNS-01
2017/12/27 14:04:13 [utility.sre.spoken.com] Could not obtain certificates
Error presenting token: Failed to determine Route 53 hosted zone ID: NoCredentialProviders: no valid providers in chain. Deprecated.
For verbose messaging see aws.Config.CredentialsChainVerboseErrors

(In the same environment, the aws cli with a --profile works fine)

There is some literature for other projects that suggests setting AWS_SDK_LOAD_CONFIG=1 in the environment will help, but it failed for me, with the error as above.

In our particular environment, we use awsmfa to generate a short-term STS token, then use it on our master aws account to adopt a cross-account role in the account that serves route53 for us.

As a workaround, I simply made a local account in our route53-serving sub-account, and expose those credentials in the environment via a wrapper script.