[baremetal] MaaS: Enable HTTP proxy

[baremetal] MaaS: Enable HTTP proxy

Instead of using NAT on the mas01 node for all cluster node outgoing
traffic, use the MaaS built-in proxy for APT traffic to leverage its
caching capabilities too.
Also enable the proxy for salt minions, so they can access public
keyservers et al.
Cleanup public DNS from kvm nodes, interferes with MaaS proxy.

Add example config for global env proxy, but don't enable it:
- default environment settings - /etc/environment (via reclass);

The MaaS proxy will not be used (at least for now) on nodes:
- cfg01;
- mas01;

NOTE: We can't yet drop the maas.pxe_nat state completely, as certain
Openstack services are still accessed via public addresses from ctl
nodes.

JIRA: FUEL-317
JIRA: FUEL-318

Change-Id: I6c5f6872bb94afb838580571080e808bc262fc68
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>