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]I5f7164f7a7ec5d4380ca22885000caa0183a0bf7

Change-Id: Ic29cb510a558ceee832fbfae7853106decffbb41
Closes-bug: 1697588
1 file changed