Updates tox.ini to use new features

tox 1.6 allows us to skip the sdist step, which is slow. This does that.
It also allows us to override the install line. In this case, it's
important as it allows us to stop getting pre-release software we
weren't asking for.

Original patch by Monty Taylor, talked about here:
http://lists.openstack.org/pipermail/openstack-dev/2013-September/015495.html

Change-Id: Idcc32f7191df85eaac35f4f07256847b677b27fe
Improve description of some commands

Change description of sample-list and statistics commands to not refer
to the meter as "this meter(s)".

Change-Id: Ib8301361f43f8e08ef9975a609548afffba1ba41
Closes-Bug: #1259853
Merge "Updates tox.ini to use new features"