Parallelized testing framework

Based on pytest-xdist. xdist needed some modifications, which will
eventually be submitted as a pull request there so we can nuke
most of this and use xdist instead.

Specify multiple appliances via multiple --appliance command-line options,
passing a different base_url to each. The test suite modules will be split
among the child processes (and thus appliances) by test module.
parallel base urls can be loaded from conf.env