Introduce role-specific NodeUserData, use for docker

Introduce role-specific NodeUserData, use for docker

Currently when the docker environments are invoked, every node has the
boot script run which replaces os-collect-config with the heat-agents
container. This should only be happening on Compute nodes currently,
and each role will be converted to heat-agents one at a time.

This change implements a role-specific NodeUserData resource and uses
that mechanism to run docker/firstboot/install_docker_agents.yaml only
on Compute nodes.

Change-Id: Id81811dbcaf0e661c3980aa25f3ca80db5ef0954