Fix OVS and LB plugins' VLAN allocation table synchronization

Fix OVS and LB plugins' VLAN allocation table synchronization

In both the openvswitch and linuxbridge plugins, if previous entries
for a physical network have been completely removed from the
network_vlan_ranges configuration variable, allocation table records
for unallocated VLANs on that physical network are now removed from
the DB at startup. The test_ovs_db and test_lb_db unit tests have also
been extended to cover this case. Fixes bug 1052289.

Test assertions that were added to the test_ovs_db unit test in
https://review.openstack.org/#/c/11388 have been added to the
test_lb_db unit test. Fixes bug 1045596.

Change-Id: I04e924603eaf0df717414c2aaa83fd203b791308