Fix context initialization for threading

[aminebouassida]

Hello,

When trying to use the package with threading, we always get the following error:

Exception in thread Thread-1:
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/threading.py", line 916, in _bootstrap_inner
    self.run()
  File "/usr/local/lib/python3.6/threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
  File "/tmp/test.py", line 4, in test
    with oc.project("test-project"), oc.timeout(10 * 60):
  File "/usr/local/lib/python3.6/site-packages/openshift/context.py", line 111, in __enter__
    if len(context.stack) > 0:
AttributeError: '_thread._local' object has no attribute 'stack'

And the root cause turns out to be that the context variable is only initialized for the main thread.
The solution was to overload the threading.local class, to force the initialization for all threads.

Kind regards,
Amine

[bradmwilliams]

Closing this PR since similar changes to these have been accepted in PR #62