Refactor minion files into single minion file

[nixjdm]

Currently there is a different minion file for each provider https://github.com/terminal-labs/sample-states/tree/master/saltstack/minions. The only difference is the few grains that are set. We're currently setting the default user to vagrant for ec2, do, and virtualbox. Once we do the same for docker and lxc (which currently use the default name root I think), we don't need multiple minion files. Thus should start using a single minion file instead of minion.[provider] files. This will need a small change to our vagrantfiles.

Prereq: #306, #307

[nixjdm]

Implementation detail: Check for and use a minion file if it exists, if not, then fall back to minion.[provider]. This will provide backwards compatibility with the corresponding change from the sample states.