(maint) Add docker specifics to git testing

[melissa]

Unfortunately there isn't a ticket for this work.

However, there are two things that are happening as a part of this
commit.

First, we don't want to try to download or unpack the runtime tarball.
If we're using docker to run the tests, the user should be pulling an
image that already has all the stuff that runtime provides. Hopefully
they're using a relatively new image that corresponds to whatever branch
they're working on. This is a big assumption, but it allows us to remove
the requirement that this be run on infrastructure that is only internal
to puppet.

Next, we allow dependencies to be installed from a directory that has
been bind mounted into the docker container. This will let us run tests
against a code base that we can be modifying as we go. We don't have to
figure out how to sync any changes up to the server that tests are
running on. They're just there! There are also some pretty big
assumptions with this, but again, it's worth the risk IMO.

I detailed some of this in comments in the code, but I'll add it here
too.

The hosts file should look something like this:

---
HOSTS:
  hostname-1:
    hypervisor: docker
    image: agent-runtime-master:latest
    docker_image_commands:
      -
      -
      -
    mount_folders:
      puppet:
        host_path: ~/puppet
        container_path: /build/puppet

This means that the hosts file that beaker-hostgenerator currently
creates for you is insufficient. You can still generate one using
beaker-hostgenerator, but you will likely want to go in and update it.

bundle exec beaker-hostgenerator -tdocker debian8-64m-debian8-64a > hosts.yaml

[caseywilliams]

Jenkins failures look unrelated -- 👍!

[melissa]

Jenkins retest this please

[melissa]

jenkins test failure is because I force pushed to my branch. Sometimes it takes a little while for it to handle that update

[melissa]

Jenkins retest this please