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
(cherry picked from commit ee5c3b0bb60918a7e59e89ceffbafa2f683b74fd)
Merge "Make plugin deallocation check optional" into stable/icehouse