Replace the usage of 'admin_manager' with 'os_admin'

In tempest, alias 'admin_manager' has been moved to 'os_admin'
in version Pike, and it will be removed in version Queens [1].

[1] https://review.openstack.org/#/c/467852/

Change-Id: I6d51cfcf2a5c931b5af2e32fa2ef153181fbca88
1 file changed