Add a node pattern for precise3k slaves.

* manifests/site.pp: Add a node pattern for slave servers which run
Ubuntu Precise Pangolin and default to Python 3.3 for the Puppet PIP
package provider.

* launch/README: Starting with precise3k slaves, individual Puppet
certificates will now be used per-server.

Change-Id: I2cde440720afd8014fc98dfd133d29a428751a9d
Merge "Add a node pattern for precise3k slaves."