Reduce the severity of dhcp related log traces

Due to timing issues, when the agent synchronizes
with the server wrt the representation of server's
and agent's networks, networks may have already
been deleted from the db. In such circumstances,
it is more appropriate to emit warnings rather than
full-blown exceptions as the agent handles these
circumstances just fine.

Close-bug: 1251874

(cherry picked from commit f1b9ac5a542a3125d757094fccda80c80c6dd420)

Conflicts:

        neutron/agent/dhcp_agent.py

Change-Id: I4b118b6087c7f42226f0d4265d2176c028d99f22
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable