Setup script and commands for installation and configuration

Implemented a setup script to install recommended Vagrant plugins,
build/install the bundled mediawiki-vagrant plugin, and interactively
configure required settings (the only truly required setting is git_user
at the moment).

Settings are defined through a basic DSL that allow for helpful
descriptions, default values, and value coercion (enforcing min/max
values, etc.).

Additional commands `vagrant config` and `vagrant forward-port` were
implemented to provide a command-line interface to defined settings and
forwarded ports, respectfully.

Removed esoteric settings box_name, box_uri, and puppet_debug. Puppet
debugging can now enabled by defining a PUPPET_DEBUG environment
variable.

Change-Id: I748655162214e05adc101e626b11b7fd91a29f22