S3 sync fails if provider.region is a variable

[spraot]

This means we have to duplicate variables in a mono-repo because the references to shared variables break s3-sync. It appears that s3-sync does not attempt to resolve the variable at all.

[williamsandonz]

This is also affecting me, basically provider.region: ${env:foo} is not resolved and the sync fails.
I've created PR here: Could we please get have this reviewed and merged?

[clay-spfmlp]

It looks like provider.getCredentials().region always returns undefined no matter what you have region set to. We are trying to deploy to aws gov cloud and it breaks the whole deployment. If we replace provider.getCredentials().region with provider.getRegion(), it works.