Display message when bootstrapping with lxd provider

[voidspace]

Partial fix for bug 1602192.

https://bugs.launchpad.net/juju/+bug/1602192

Display a custom message when bootstrapping with the lxd provider, pointing the user to configuration information.

The reason we're displaying an advisory rather than making the changes from Juju is that these settings need to be set on the host machine and they need system privileges plus a reboot. We could do it at Juju install time, but not at bootstrap time as we only have user privileges and we shouldn't be making intrusive system changes just because the user has installed Juju.

QA
Bootstrap with the lxd provider. It should look something like this:

$ juju bootstrap foo lxd
Creating Juju controller "foo" on lxd/localhost
Looking for packaged Juju agent version 2.0.0 for amd64
No packaged binary found, preparing local Juju agent binary
To configure your system to better support LXD containers, please see: https://github.com/lxc/lxd/blob/master/doc/production-setup.md
Launching controller instance(s) on lxd/localhost...

• juju-6e02cd-0
  Fetching Juju GUI 2.2.0

[voidspace]

Suggestions for useful tests welcomed.

[macgreagoir]

(IANAGR) LGTM.

I think it's the case that we don't want to make these changes to the system config as part of provisioning because they require a reboot. If that's true, perhaps we should mention that in the commit message, for posterity.

[voidspace]

$$merge$$

[jujubot]

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