Update Python package requirements

With the switch to oslo.logging, the logstash logging option was
removed, so no need for python-logstash.

We can also get by with Babel>=0.9.6 which is readily available
with the default python-babel Ubuntu package. 1.3 is very new and
not yet available in PPAs, so no need to complicate our requirements.

Change-Id: I42c285a9084feb5c603c7f97136f87a88af38846