3857: code review feedback

3857: code review feedback

* Set default timeouts to (20, 300) for proxies and (2, 300) for all
  else.
* KeepService:
** Removed unused 'timeout' argument to __init__()
** Trimmed unnecessary exceptions in last_status()
* Tests:
** test_collections.py: changed mock_api_call return value back to
   fake_httplib2_response
** Flattened test layers to eliminate KeepClientRetryGetTestMixin and
   KeepClientRetryPutTestMixin
** Moved proxy timeout tests to
   KeepClientServiceTestCase.test_proxy_get_request and
   .test_proxy_put_request
* Removed excess blank lines in docstrings