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
Make plugin deallocation check optional

The fix for memory leakage in the related bug added a check for plugin
deallocation that performed a call to gc.collect() after every test.
This had the side-effect of increasing test execution time by ~50%, so
this patch makes the check optional via an environment variable
(OS_CHECK_PLUGIN_DEALLOCATION).

It may make sense to create a periodic job that runs with the check
enabled, but otherwise the check can be used by developers for
debugging purposes.

Change-Id: I9ebe663abbc4e4ff3a62d807d5a3230c2ecccd07
Related-Bug: #1234857
Merge "Make plugin deallocation check optional"
Merge "ML2 VxlanTypeDriver: Synchronize of VxlanAllocation table"