Stop logging unnecessary warning on context create

The context was previously logging at the 'warn' level when unknown
kwargs were being passed to its __init__().  Since the agents were
passing tenant=None with each rpc request, this was generating an
unreasonable amount of log chatter that would not be useful to an
operator.  The fix is to log at the debug level instead so that
the operators don't see the output by default but developers can
still choose to.

Change-Id: I5c328f628c597eb949c1fe67b23120d2b5d1c7da
Related-Bug: #1254530
Partial-Bug: #1255441
Merge "Stop logging unnecessary warning on context create"