Enhance list_group_users in GroupApi.

If there is no member in the group.
When get the group users, the varible "member" is an empty dictionary.
It will crash at "member[self.member_attribute]".
This commit add a judgment to determine the member is empty or not.

Closes-Bug: #1259426
Change-Id: Ic20549204bcc35f71464b858f5ca900a4bbfe049
Cleanup of new credential_api delete methods

Based upon the feedback on the credential_api.delete_credentials_for_user
and credential_api.delete_credentials_for_project implementations, apply
some cleaner default logic and comments/docstrings.

Change-Id: I6d1abe71931bce82c122d6cce9a94beccd8ffcfb
bp: assignment-controller-first-class
Merge "Enhance list_group_users in GroupApi."
Merge "Cleanup of new credential_api delete methods"