support local debug logging

Add OS_DEBUG as an environmental variable that will let you run
locally with debug logs when trying to debug a particular unit
test. Inspired by similar function in the neutron base tests.

History: I70790aa39c0774726de71e5fa5751c45e7e34ffd was enabled on
March 10th to address 2 issues, make it possible to see DEBUG logs
during test runs, and test that the LOG.debug functions that we happen
to execute as a side effect of unit tests did not have syntax errors
(it's important to note it was not guarunteed coverage for all debug
messages).

The net effect was largely a DDOS on the testing system. This
generated a 190MB subunit_log per test run. It meant on failures in
Nova unit tests now generated such an enormous dump that they became
non indexable in elastic recheck (it overloaded the indexers), so the
bug classification system was blinded to any nova unit test bugs.

It was thus reverted in: Ia1cfc3d874ddf4f8ee2037b487d22f968585854e

This patch solves the first concern, which is developer targetted
testing. It mirrors what is done in Neutron for cross project
consistency. There was a competing approach in requiring OS_LOG_LEVEL
to be set, however it had the same problem as the initial patch,
defaulting to DEBUG means that it's a DOS (at 15k unit tests, that
just can't be the default). Given that Neutron already established
some conventions here, using existing conventions also seems better.

This *does not* address concern #2. That is expected to be done in a
separate patch using an ephemeral string buffer logger. However
getting that correct has proved tricky enough that the approach to the
problem was:

 1. revert - Ia1cfc3d874ddf4f8ee2037b487d22f968585854e
 2. add targeted debug logging (this patch)
 3. add back side effect debug testing - being worked by joe

Change-Id: I04ff2f0650f1e6035d8de5c5802b96ce4294891b
(cherry picked from commit 31169bb26fc0a39486ef6cd6a5a154d570186886)