Bug 1055793 - Use runner on OS X; r=dustin

Installs runner along with a launchd init config which runs runner as the cltbld user.
Runner had previously assumed it would be run as root, by init.d/upstart, and
had opted to use 'su -' prior to starting the buildslave. Because starting
runner as cltbld is possible, and desirable for the resulting environment,
the Linux approach fails as it produces a user environment with no display and an incomplete path.
To fix this the runner buildbot task's template was modified to allow custom
buildslave commands to be set via puppet variables.
Merge default -> production

--HG--
branch : production