Use Python 2 when installing zuul on workers

* nodepool/scripts/prepare_node.sh: The py3k-precise nodes use a
virtualenv which defaults to python3.3, and zuul has at least one
dependency which fails to install in this situation (python-daemon
tries to use a non-Py3K compliant print syntax in its
version_info.py module). By telling virtualenv to use python2
instead we sidestep this issue.

Change-Id: Idfa70dee91494da83380fa0c0a1f8427659f6ce2