Add single-use py3k-precise nodes

* .../files/nodepool/scripts/prepare_node.sh: Add parameters for
Python 3 and PyPy settings for use by the py3k-precise node type.

* .../files/nodepool/scripts/prepare_node_py3k.sh: Wrapper to enable
the Python 3 and PyPy support toggles in prepare_node.sh.

* .../manifests/single_use_slave.pp: Add the Python 3 and PyPy
support variables, passing them through to the Jenkins slave class.

* .../templates/nodepool/nodepool.yaml.erb: Add a new node and image
for py3k-precise in a suitably small quantity for initial testing.

Change-Id: Id2438c17ea59ae9123a841fd0b348728c6467529
Merge "Add single-use py3k-precise nodes"