contint: install Zuul on all CI slaves

Zuul will have a command line utility to fetch out multiple
repositories.  Thus install Zuul on all CI slaves to provide the
zuul-cloner utility.

The package python-pip is needed by both Zuul source code installation
and the contint::packages::labs class.  So I wrapped the package
definition with ensure_packages() from stdlib to avoid a duplicate
definition.

Change-Id: Ib1c1fc777bdcba90b89ad4f3d46430ae8e7daadf