tempest: start using get_configured_admin_credentials

Tempest removed get_configured_credentials in [1].
This patch aims to update the way we get admin credentials in Tempest
client code.

[1] https://github.com/openstack/tempest/commit/bc0a7a6d8022c7240f5f1f80c0c93b43e9037ad4

Change-Id: Ic97c011f8beec8df4253949285794be21b10f7cf
1 file changed