Don't use credential provider if credentials are given

If the aws_* params are provided in a create_client() call,
we still call session.get_credentials(), which is an unnecessary
call.  We should only invoke the credential lookup process if
explicit credentials are not provided in the call to
create_client().
Merge branch 'fix-cred-prov' into develop

* fix-cred-prov:
  Don't use credential provider if credentials are given