CW client creds refresh

[brianterry]

Issue #, if available:

Description of changes: This change refreshes provider creds during runtime initialization Test: $ go test ./... ok github.com/aws-cloudformation/cloudformation-cli-go-plugin/cfn 0.688s ok github.com/aws-cloudformation/cloudformation-cli-go-plugin/cfn/callback (cached) ? github.com/aws-cloudformation/cloudformation-cli-go-plugin/cfn/cfnerr [no test files] ok github.com/aws-cloudformation/cloudformation-cli-go-plugin/cfn/credentials (cached) ok github.com/aws-cloudformation/cloudformation-cli-go-plugin/cfn/encoding (cached) ok github.com/aws-cloudformation/cloudformation-cli-go-plugin/cfn/handler (cached) ok github.com/aws-cloudformation/cloudformation-cli-go-plugin/cfn/logging (cached) ok github.com/aws-cloudformation/cloudformation-cli-go-plugin/cfn/metrics (cached)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.