Swarm flavor revamp

[chungers]

This PR overhauls how the Swarm flavor plugin works. Before, the swarm flavor had to be run separately between the manager and workers -- where the manager is a single node, configured by a vanilla plugin, while the workers are configured separately using the swarm flavor itself. Now the swarm flavor plugin

• relies heavily on templates - one for worker and one for manager to give the user the ability to customize what additional tasks are to be run in the init scripts.
• uses the manager's multi-group handling to make managing two different groups in a single JSON configuration possible.
• the templates allows the user to control the init for the first node (where swarm init is needed) and the manager followers and the workers. New template functions are introduced so that the user can essentially program how the init sequence for the different nodes should work.
• removes the need to bootstrap initial leader manager node via the vanilla plugin.
• a single configuration, a single commit, can provision both the manager and worker groups.
• a different version of using actual get.docker.com script to install Docker is also provided -- as compared to the previous demos of using pre-installed docker on vagrant boxes.

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "swarm-flavor-revamp" git@github.com:chungers/infrakit.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354041880
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.