MidoNet: Added support for the admin_state_up flag

This commit enhances the port, bridge and router resources with the
admin_state_up flag in the MidoNet plugin.

Implements: blueprint midonet-admin-state
Change-Id: I67f4f9ad4099a05f8161aae79331ebba84f561b8
Merge "MidoNet: Added support for the admin_state_up flag"