Add post-mortem debug option for tests

Add post-mortem debug option for tests

Post-mortem debugging, the ability to drop into a debugger with the
execution state that triggered the exception, is very useful in
diagnosing failure conditions.  Our previous test runner, nose,
provided the ability to enable post-mortem debugging on test
failures (via --pdb-failure) and errors (via --pdb).  testr
lacks these options at present, so this change adds support
for enabling post-mortem debugging via an environment variable.
All test-triggered exceptions will result in a post-mortem debugger
being invoked if OS_POST_MORTEM_DEBUG is set to "1" or "True".

Implements: blueprint neutron-pm-debug-on-test-failure
Change-Id: Iddbe1335b059d062c0286df2ad27aef7728461b7