This adds a simple role to install Zookeeper.
Add an option to nodepool-base to use this role to install Zookeeper.
Use this in the nodepool-builder gate testing where we are just
validating that the nodepool-builder container starts and is ready to
accept connections. It needs a zookeeper to talk to, even though it
is not going to do anything.
Change-Id: I4ae89a51e454be4ee53ad4e04407162aaa8d9f9a