Fixes copy operation for Swift snapstore

[plkokanov]

What this PR does / why we need it:
Fixes the retrieval of credentials when using the copy operation with the Swift snapstore.

Without this fix we get the following error when trying to copy backups:

WARNING: Package "github.com/golang/protobuf/protoc-gen-go/generator" is deprecated.
	A future release of golang/protobuf will delete this package,
	which has long been excluded from the compatibility promise.

time="2022-05-06T10:29:51Z" level=info msg="etcd-backup-restore Version: v0.15.2"
time="2022-05-06T10:29:51Z" level=info msg="Git SHA: c9b5c3d0"
time="2022-05-06T10:29:51Z" level=info msg="Go Version: go1.17.9"
time="2022-05-06T10:29:51Z" level=info msg="Go OS/Arch: linux/amd64"
time="2022-05-06T10:29:51Z" level=fatal msg="Could not get source and destination snapstores: could not get source snapstore: Missing input for argument [auth_url]"

This is because the getClientOpts function was exiting too soon if isSource==true and was never trying to read the file containing the source store credentials.

I've added the following code for backwards compatibility:

return &clientconfig.ClientOpts{
	EnvPrefix: envPrefixSource,
}, nil

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

Fixed retrieval of credentials during copy operation for backups stored in Swift snapstore.

[plkokanov]

/cc @ishan16696
Could we also cherry pick this for a hotfix?

[ishan16696]

This is because the getClientOpts function was exiting too soon if isSource==true and was never trying to read the file containing the source store credentials.

yes, I also noticed this when I was working on the secret rotation PR and wondered why you guys are reading secret this way when isSource=true, sorry I forgot to inform you.

if isSource {
		return &clientconfig.ClientOpts{
			EnvPrefix: envPrefixSource,
		}, nil
}

[ishan16696]

Could we also cherry pick this for a hotfix?

ok, do you want a hotfix release for backup-restore v0.15.3 ?

[ishan16696]

LGTM!!

[plkokanov]

Hey, @ishan16696 I see that this has not been cherry picked on 0.16.0 release which has the same problem. Can we cherry pick it there as well asap?