Fixed lp:1401130 - networker is disabled for joyent environments

Networker was causing problems on Joyent environemnts.
Since http://pad.lv/1401130 is a CI blocker, this is
a temporary fix that disables the networker for Joyent.

I've filed a separate low-priorty bug http://pad.lv/1401423
to propely fix the networker for joyent, when possible.

Live tested on Joyent.

(Review request: http://reviews.vapour.ws/r/623/)