Replace openvswitch plugin's VlanMap with vlan_ids DB table.

Replace openvswitch plugin's VlanMap with vlan_ids DB table.

Fixes bug 1023167.

The openswitch plugin's in-memory VlanMap is replaced with a vlan_ids
DB table similar to that used by the linuxbridge plugin. This will
prevent conflicting VLAN assignments if multiple server replicas are
run, and also sets the stage for phase 2 of the provider-networks BP
implementation that will add support for multiple physical
networks.

Unlike with the current linuxbridge plugin, the contents of the
openvswitch plugin's vlan_ids table are properly updated at startup in
case the vlan_min or vlan_max configuration variables have changed.

A new test_ovs_db test case has been added.

The primary key of the vlan_bindings table is changed from the vlan_id
to the network_id, which is now a foreign key, and network deletion is
now properly handled.

The net_id has been removed from the VlanIdInUse exception, requiring
a minor update to the linuxbridge plugin. The new NoNetworksAvailable
exception, with ResourceExhausted as its base class, is returned when
no more VLANs are available.

Change-Id: I65a2347dea5366cc0d15d98a88f40e42e1a45f4c