Injectable statsd client (#7138)

* Add ability to inject statsd client; some py test/reqs updates

- Updated the metrics logger to allow construction with an existing
statsd client, so that it can be configured by external systems or libs.
- added requirements to requirements-dev.txt which are needed to run
  tests-eg coverage, nose
- removed dependency on mock lib, it is in python stdlib now
- updated tox.ini to remove the now-superfluous deps

* add license to test file, and remove blank line at EOF

tests/access_tests.py

@@ -17,8 +17,7 @@
 """Unit tests for Superset"""
 import json
 import unittest
-
-import mock
+from unittest import mock
 
 from superset import app, db, security_manager
 from superset.connectors.connector_registry import ConnectorRegistry