Fixed lp:1395908 - disabled NICs from lshw stay disabled

With this fix the MAAS provider takes the disabled="true"
attribute of <network/> elements into account when deciding
which should be the primary network interface on the machine.
The end result is juju-br0 will be configured to use the first
enabled network interface.

Live tested on MAAS.

(Review request: http://reviews.vapour.ws/r/558/)