Move cred_provider abstract class to tempest lib

This commit migrates the cred provider abstract class into tempest lib.
This interface has been very stable and doesn't have any external
dependencies as it's just a building block for a cred provider class.
This also doesn't have any unit tests because it's just an interface
definition and doesn't actually do any work.

Change-Id: Ib5f8ef2728368c077666a3ca221cf659412e5f47
5 files changed