Install arcanist for developers

Installs libphutil/arcanist via 'arcanist' role to /vagrant/phab.
This is configurable via hiera.

Arcanist can be invoked from /vagrant/phab/arcanist/bin/arc and
is available as `arc` in the VM.

Change-Id: I898fce744518877eb8db3299f51f1c6e05c6aeac