Minimize ovs l2 agent calls to get_vif_port_set()

Minimize ovs l2 agent calls to get_vif_port_set()

The ovs l2 agent was previously calling get_vif_port_set() on the
integration bridge once per rpc_loop() iteration and then again in
the periodic _report_state() call that returns the current device
count to the neutron service.  Since get_vif_port_set() is an
expensive call (relying on shell commands) and since there
is minimal risk associated with reporting stats that are a few
seconds old, this patch caches the device count for reuse by
_report_state().

Partial-Bug: 1177973

Change-Id: Ice73384ed1ba1e97120028cd0a9bff94a62a41a4