Add Slurm Docker tests

[simonbyrne]

Start on #105, alternative to #125

Uses container from https://github.com/giovtorres/docker-centos7-slurm

[kescobo]

Oh, awesome - this will be great!

[simonbyrne]

Note that this runs both the compute daemon and the scheduler in the same container, so it may not be the most realistic use case.

For something more advanced there is https://github.com/giovtorres/slurm-docker-cluster, but I don't really know enough about containers to figure out how to use it. @kleinhenz used something like this in the CI for https://github.com/kleinhenz/SlurmClusterManager.jl

[kleinhenz]

My testing setup uses https://github.com/kleinhenz/docker-slurm-cluster which was adapted from https://github.com/giovtorres/slurm-docker-cluster by removing as much complexity as possible (most notably the database features). I'm not sure how important it is to have the compute nodes isolated but I guess it would make it a bit more realistic.

[kescobo]

Is there anything keeping this from being merged? Would be really nice to have this going forward, I think.