Make nova-network a "conductor-using service"

In order for a service to reliably use conductor at startup, it must
wait for conductor to become available before making any requests.
This is indirectly controlled by the "db_allowed" flag to the service
startup. This should be equivalent to the "use_local" setting in
the conductor group, so this patch sets this for network so that a
subsequent patch can introduce a conductor-using call in init_host().

Note that compute was incorrectly passing this flag always as False,
so this patch fixes that up as well.

Related to blueprint nova-network-objects

Change-Id: I9a98fb943895d0d2d359e0d9d38f09fb0f6ddd5a
Merge "Make nova-network a "conductor-using service""