Fixed lp:1442257 - port of #2366 to 1.25

[dimitern]

This includes 2 fixes for 1.25:

• http://pad.lv/1442257 - instead of inheriting LXC containers' host's
  primary NIC's MTU value for container NICs, now we have an optional
  integer environment setting "lxc-default-mtu", which when set to a
  positive number will cause all LXC (but not KVM) NICs to use that
  MTU. This is a straightforward port of #2365 to 1.25.
• Related, but not a fix for http://pad.lv/1443942 (originally proposed
  as #2190 but it didn't land in time) - only add iptables SNAT rules
  on the container hosts for EC2 environments, as this breaks MAAS and
  it's not needed (fallout from the original fix #2071 for bug
  http://pad.lv/1442801).

Live tested on MAAS and EC2 with the following configurations:

• bootstrap on trusty, add a precise node
• bootstrap on precise, add a trusty node
• in both cases above, do a test with and without the address-allocation
  feature flag
• deploy workloads and relate them in LXC and KVM

More unit tests around the enableNAT logic will be added later,
as described bug lp:1443942.

[themue]

Sadly no constant when this setting has been introduced.

[dimitern]

Yeah :/

[themue]

LGTM 👍

[dimitern]

$$fixes-1442257$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Does not match ['fixes-1457031']
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/3347

[dimitern]

the blocker is Fix Committed, so $$JFDI$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju

[jujubot]

Build failed: Does not match ['fixes-1457031']
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/3348

[dimitern]

$$JFDI$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju