Remove the warning for Scheduling Network

when a new port is created, the schedule_network in agentschedulers_db is called.
Since this network may already be hosted, it will directly return None in dhcp_agent_scheduler,
which cause the agentschedulers_db.schedule_network log a warning message.

This is really annoying and making users confused.
And, real warnings are already addressed in the real scheduler function.
which is neutron.scheduler.dhcp_agent_scheduler.ChanceScheduler.schedule by default.

Cherry-picked from 3ef877fc5a669659c2fa4bb2fc416d4a473c6ac5 in master.

Change-Id: I11bdbe47c2cc0fc45fb143970fb6762678bc27e5
Closes-Bug: #1192786
Merge "Remove the warning for Scheduling Network" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable