Use Salt to install the Salt master.
The Salt master will not get restarted automatically,
neither on configuration changes nor on package changes.
Instead, it will need to be manually restarted to avoid interrupting
the in-progress highstate as well as allow for proper update ordering.
(Salt masters must be updated before Salt minions.)

Use Jinja to generate the config file YAML directly, preventing
typos in the master config file and providing access to master
configuration options in the SLS file (e.g. the file_roots locations).

Previously, the minion config was used to specify that a local set
of file trees should be used. However, in order to test that the Salt
master states function, and to more closely match the actual deployment
of our minions, amend the minion config to point to a localhost master.
Additionally, amend our Travis config and Vagrantfile to explicitly
pass options to set the local file and pillar trees.
Only sync them via Vagrant shared folders for the Salt master, which
should have them available to allow debugging. Because using Salt is
the only way to learn which minions (ids) would have a Salt master
running on them, hardcode the salt-master\d+ regex into the Vagrantfile.

In the future, this will allow adding a smoke test that check if the
master can test.ping a local minion (on the same VM). However, this
requires the minion to be started, which requires that the Salt minion
also be salted, (which requires this PR to backport the launchctl.py
execution module).

Use gitfs backed by the saltfs repo (https://github.com/servo/saltfs/)
as the main source for the Salt file tree.

For local testing, additionally configure a local directory in /tmp as
a Salt file tree root, and give it precedence over the gitfs tree.
This allows temporarily cloning the saltfs repo into that directory
and makes local changes to iterate on them.
Add an ADMIN_README file with some ground rules about proper usage of
the directory; this file should be set as immutable, but Salt does not
yet have a module/state for this.

Also update the buildbot slave state to use the
now-working service state on OS X, and update the
name of the service from 'buildslave' to
'buildbot-slave' for better distinguishability.

Add a note to README.md about licensing of files
taken and/or modified from the main Salt repo.

The backported versions support old and new pips more robustly without
depending on pip internals.

Recently (in version 8.1.2), pip changed their internals in a
non-backwards compatible way, breaking the Salt pip modules.
Salt used to depend on these internals, but has since been updated
to no longer depend on them as they have no compatibility guarantees.
We have been encountering issues where Salt is finding a new pip
version, even though the installed pip is old (6.1.1). To work around
this issue, backport updated modules that handle both old and new pips.