Added disable-network-management environment flag

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.
 * The networker worker is always run in safe mode, so no subsequent
   changes to the network config are made, or and ifup/ifdown commands
   will be run.

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.
It does indeed and once this lands, when "disable-network-management" is true, the environ capability for "requires safe networker" should always return true for all providers (i.e. first check if it's disabled, then the rest of the conditions).