Fix eventlet monkey patch and threadlocal usage

* according to the docs
  http://eventlet.net/doc/patching.html#eventlet.patcher.monkey_patch
  the monkey patching should be done as early as possible. It looks like
  this requirement wasn't satisfied before and probably it was the
  reason for several issues with threading;
* all tests are now using base savanna tests with test context available
  to use;
* threadlocal usage was fixed by using monkey patched threadlocal in the
  correct way - as a global storage;
* volumes test was fixed to use test thread group that executes all
  stuff consecutive, helper added to the base tests class.
* sahara.context now uses only 'greened' system modules. That is needed
  for context_test to pass, as it requires 'green' context and we don't
  do monkey patching in tests

Co-Authored-By: Dmitry Mescheryakov <dmescheryakov@mirantis.com>

Change-Id: I3d31ceb4b9c867d4875bfca948beb52260ee8957
Merge "Fix eventlet monkey patch and threadlocal usage"