Abstract sync_allocations

Abstract sync_allocations

Currently gre/vxlan type drivers have specific sync_allocations
implementations to perform the same feature. This change abstracts
(vxlan) implementation and moves it to TunnelTypeDriver[1] in order to
share implementation between gre/vxlan; this change also adds some
unittests.

Current gre/vxlan and new implementations of sync_allocations are not
Galera-compliant with multi-writers, a follow-up will update it to
fully support Galera.

[1] neutron.plugins.ml2.drivers.type_tunnel

Change-Id: I188a7cf718d811084475f6783d844588de5d60ea