Replace the usage of 'admin_manager' with 'os_admin'

Alias 'admin_manager' has been moved to 'os_admin' in version Pike,
and it will be removed in version Queens. This patch is to replace
the usage of 'admin_manager' with 'os_admin' which mainly used in
Tempest scenario tests.

Change-Id: I2ee802a2dc20eb4f065ecccec127f817b1d6743c
6 files changed