AWS_PROFILE support

[asafdav]

First of all, thanks for this amazing terraform provider!
In our use case, we have different credstash instances spread over different accounts, I had to add profiles support and I think that other people can benefit from that.

It should solve #3

[tmichel]

Thanks for the PR. It looks good.

Please fix the build. It seems that something got mixed up and the compilation fails.
Also please squash your commits into a single one with a descriptive commit message. Thanks!

[tmichel]

If this should come from AWS_PROFILE environment variable that please use the schema.MultiEnvDefaultFunc([]string{"AWS_PROFILE"}, "default") default value provider so that environment variable is picked up by default.

[asafdav]

If you set AWS_PROFILE environment variable it drives Terraform itself nuts + I think it's just nicer to set the provider as:

provider "credstash" {
  table  = "some-table"
  region = "some-region"
  profile = "some-profile" 
}

[asafdav]

@tmichel thanks for the quick workaround.

1. I think that the build fails because of this import https://github.com/asafdav/terraform-provider-credstash/blob/e16edd4ae6d111053cb6ff749982d82a71052edd/datasource_secrets.go#L10
   instead of fetching my updated version of "credstash" it takes your original one, I forked the entire project and changed it to asafdav/* and it worked
2. I can squash my commits but it's not needed you can squash the PR when you merge it using Github, isn't it cool ?
   

[tmichel]

Cleaned up the code (gofmt) and pushed to master: 2433b14.

Thanks again!