AWS: extract credentials from awscli config

[kdomanski]

Currently the AWS code of KubeOne expects the AWS credentials (e.g. for the machine controller) to be contained within the AWS_* environmental variables and picks them up using custom code. A user is likely to have these credentials in their AWS CLI config file instead, especially since most provisioning tools, e.g. Terraform or Vagrant are perfectly capable of accessing these - therefore KubeOne should also have this capacity, and it should follow typical conventions e.g. for picking the profile.

Acceptance criteria:

• for AWS clusters KubeOne first attempts to fetch env-var-based credentials using github.com/aws/aws-sdk-go/aws/credentials.NewEnvCredentials()
• If that fails, it should attempt to extract them from the config file's current profile using github.com/aws/aws-sdk-go/aws/credentials.NewSharedCredentials("", "")
• if that fails too, the failure should be immediate and KubeOne should not attempt to deploy machine-controller with empty credentials

[xmudrii]

I want to give this a try.

/assign
/lifecycle active