contint: on slave labs, install tox from pip

Puppet supports fetching packages from the python pip repository. The
version of tox in Ubuntu Precise is way too old and backporting the
packages requires to bring a lot of python dependencies.

The contint::packages::labs is harnessed to prevent its usage on
production servers.  ensure => present to prevent puppet from spamming
pip as well as unwanted upgrades.

bug: 44443
Change-Id: I714424de4e6c615a7189681efe404d1476a3c3a2
Merge "contint: on slave labs, install tox from pip" into production