nagios: Disable the SSH checks on localhost

The Nagios Docker container won't have SSH access enabled.

The irregularity in tabs vs. spaces occurs upstream.
salt: Add a generic Salt image to be shared between minion and master images

The master image is coming with the next commit.
salt-minion/README.md: Update links
salt-master: Add a Salt master image

You could use nsenter [1] instead of 'docker exec', but we should
point folks at the builtin approach.

[1]: https://github.com/jpetazzo/nsenter