Whitelist external mysql-connector-python req

Pip 1.5 changes how it deals with external packages:

http://www.python.org/dev/peps/pep-0438/

This is affecting OpenStack projects all over:

https://bugs.launchpad.net/openstack-ci/+bug/1266513

Applying this temporary fix for the mysql-connector-python
package.

Change-Id: I6f5a9f97a42336dc94c66b09ba81dd68bdab7803

tox.ini

@@ -5,7 +5,7 @@ skipsdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install -U {opts} {packages}
+install_command = pip install --allow-external mysql-connector-python --allow-insecure -U {opts} {packages}
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 commands = python setup.py testr --slowest --testr-args='{posargs}'