ML2 VxlanTypeDriver: Synchronize of VxlanAllocation table

At neutron startup VxlanTypeDriver syncs tunnel range from conf
to DB. In case multiple servers deployment restarting several
servers at the same time could lead to DB exceptions being thrown.
Need to synchronize between neutron servers by locking VxlanAllocation
table.

Change-Id: Idf9908f039070b9194612484603f592f1a8d74b8
Closes-Bug: #1307295
Merge "ML2 VxlanTypeDriver: Synchronize of VxlanAllocation table"