Enable services on agents with admin_state_up False

Enable services on agents with admin_state_up False

Previously when admin_state_up of an agent is turned to False,
all services on it will be disabled.
This fix makes existing services on agents with admin_state_up
False keep available.
To keep current behavior available the following configuration
parameter added.

* enable_services_on_agents_with_admin_state_down

If the parameter is True, existing services on agents with admin_state_up
False keep available. No more service will be scheduled to the agent
automatically. But adding a service to the agent manually is available.
i.e. admin_state_up: False means to stop automatic scheduling under the
parameter is True.
The default of the parameter is False (current behavior).

Change-Id: Ifba606a5c1f3f07d717c7695a7a64e16238c2057
Closes-Bug: #1408488