Avoid all() query when generating tunnel id

[Salvatore Orlando]

There is no need for: https://github.com/openstack/quantum/blob/master/quantum/plugins/openvswitch/ovs_db_v2.py#L371
Sql Alchemy could be used to extract directly the max value from tunnel.id, thus saving time especially when several hosts are employed.

This was observed by Jason in a recent review.
It's a negligible improvement, but a very easy one.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/27875

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/27875
Committed: http://github.com/openstack/quantum/commit/bde296d1040a6c1397faebf80f0ba164bed2c937
Submitter: Jenkins
Branch: master

commit bde296d1040a6c1397faebf80f0ba164bed2c937
Author: Salvatore Orlando <email address hidden>
Date: Wed May 1 02:57:55 2013 +0200

    Use sql alchemy to fetch a scalar for the max tunnel id

    Bug 1174998

    Change-Id: I77203c81c2c910a1a601416efc4567f1320a2eef