popcorn: migrate to nomad

[classabbyamp]

this leaves only buildbot outside of nomad, I think.

should be deployed/merged in parts:

1. merge and build container (and update the tag in nomad)
2. apply addition of popcorn host volume in ansible
3. copy all popcorn data from the current location to the volume
4. decommission ansible-based popcorn
5. run the nomad jobs

testing:

• ran all components in concert in docker (manually), which worked perfectly
• ran nomad job plan for each job (failed to place because the host volume doesn't exist yet, but no other errors)

fixes #76