Move test_describe_table to stable

Related to: blueprint move-passed-tempest-tests-to-stable

Change-Id: I7160f33324239af6d2c08aea29d0c47e99e72baf
Added function for creating keystone credentials

Change-Id: Ic5188f3ead1587f20ae6177debff505c9688cdb3
Implements: blueprint magnetodb-keystone-devstack