refactor v3 cred tests to work with pre-prov

This commit aims to use existing credentials to execute tests:
  * test_credentials_list_delete
  * test_credentials_create_get_update_delete
so that users with a cloud using the pre-provisioned credential
provider can execute these tests.

Change-Id: Ida753ad4f550eeeec47b6f34b73c936b0eab3d71
Partial-Bug: #1714277
1 file changed