unmanage password for admin keystone user in nailgun

Setting keystone_user 'admin' to be unmanaged allows
a user to change his or her password to another password
and expect it to remain in tact after reapplying puppet
during a Fuel Master upgrade scenario.

Change-Id: I16a2c1fae2cc7916a9afc9847fcd03b20f431258
Closes-Bug: #1400701
Merge "unmanage password for admin keystone user in nailgun" into stable/6.0