Installing unnecessary LXC files on compute nodes

[Paul Halmos]

The setup-hosts.yml playbook installs all LXC files, container images, and configures the container on compute nodes where there are no containers running or needed. This adds a significant amount of time to upgrade and/or deploy large environments.

[Kevin Carter (kevin-carter)]

Sadly this is not something that is easy to fence and programatically set based on general inventory. The plays have been built such that they allow all hosts within an environment to serve any purposes at any time and by not including compute_hosts in the execution group we'd essentially make it the default that no compute host could ever be used for anything other than nova compute purposes (as far as LXC is concerned). However, we have thought about the case where a deploymer may not want to have LXC bits installed on specific hosts or on groups of hosts. Within the `lxc-host-setup.yml` play you can set the execution group by defining the variable "lxc_host_group" [0]. This can be set on the command line for a one time use or can be defined within your `user_variables.yml` file. If you're sure that a node within the nova_compute group will never serve any other purpose than compute you can define the ``lxc_host_group`` like so:

  lxc_host_group: 'hosts:!nova_compute'

With that variable set in your `user_variales.yml`file or in command line using the "-e" switch the compute hosts will be skipped and no LXC target bits will be deployed to that node.

[0] - https://github.com/openstack/openstack-ansible/blob/master/playbooks/lxc-hosts-setup.yml#L17

[Jesse Pretorius (jesse-pretorius)]

This has been resolved in the master branch by the following reviews:
 - https://review.openstack.org/363395
 - https://review.openstack.org/364530

A combined backport has been implemented for Mitaka:
 - https://review.openstack.org/363759