Update configuration docs wrt admin credentials
Update configuration docs wrt admin credentials

Configuration docs are slightly dated now wrt admin credentials
and credential configurations in general. Fixing that.

Change-Id: Id176202d73683ef3adbcd93a77d90be6a5206f37

masayukig/tempest,Tesora/tesora-tempest,cisco-openstack/tempest,openstack/tempest,Juniper/tempest,sebrandon1/tempest,cisco-openstack/tempest,bigswitch/tempest,openstack/tempest,masayukig/tempest,vedujoshi/tempest,sebrandon1/tempest,bigswitch/tempest,vedujoshi/tempest,Tesora/tesora-tempest,Juniper/tempest