Adds an output for hostnames

[stevendborrelli]

• Installs cleanly on a fresh build of most recent master branch
• Upgrades cleanly from the most recent release
• Updates documentation relevant to the changes
• Rebases cleanly onto the latest master

The list of hostnames can be sent to external dns
modules for creating records.

[BrianHicks]

How will this be used? Just zipped with the public IPs for the record creation?

[stevendborrelli]

It's used with external dns providers, similar to how we use openstack.

[ryane]

aws build passed, output lgtm:

$ terraform output -module control-nodes hostname_list
resching-aws-control-01,resching-aws-control-02,resching-aws-control-03

$ terraform output -module edge-nodes hostname_list
resching-aws-edge-01,resching-aws-edge-02

$ terraform output -module worker-nodes hostname_list
resching-aws-worker-01,resching-aws-worker-02,resching-aws-worker-03,resching-aws-worker-04