Add Python 3.4 and PyPy to Ubuntu Trusty slaves

When building Ubuntu 14.04 LTS workers, install the python3-all-dev,
python3.4 and pypy packages so that they can be used by future jobs.
This is in preparation to deprecate the special "precise-py3k"
nodepool node types we maintain for py33 and pypy tox jobs.

Change-Id: If6543e75eda5822d645164636720f6c597e07729
Merge "Add Python 3.4 and PyPy to Ubuntu Trusty slaves"