Backport changes to allow network management to be disabled

Backport from trunk:

A new environment config flag "disable-network-management" added,
which is false when missing and by default, but when it's "true":
* MAAS provider is not creating a network bridge or bringing eth0 up and down during cloudinit

This is intended as a "safety switch" to workaround a few bugs happening
when using Juju on MAAS mostly, until Juju's network model works and integrates
better with MAAS. List of related bugs:

    http://pad.lv/1361374 - maas provider assumes machine uses dhcp for eth0
    http://pad.lv/1353008 - MAAS Provider: LXC did not get DHCP address, stuck in "pending"
    http://pad.lv/1345433 - cloud-init network error when using MAAS/juju
    http://pad.lv/1341524 - juju/MAAS deployed host with bonding configured via preseed missing eth0 from bond on first boot

In any cases where MAAS and Juju are both trying to manage networking
on machines, set "disable-network-management: true" in your environments.yaml
file before bootstrap.