Updated dynamic credential support for funtional test.
Updated dynamic credential support for funtional test.

Now devstack does not setup the deprecated configured cred on Tempest
which is being used here. - https://review.openstack.org/#/c/256164/

get_configured_credential use the deprecated cred and these tests use
that for initiating Manager.

Change-Id: I89713806cb98abaca29b24de585a77f75602bcd7
Closes-bug: #1531049

openstack/mistral,openstack/mistral,StackStorm/mistral,StackStorm/mistral