Cannot Pre-emptively Set ext-network via snap config

[Pen Gale]

It should be possible to set ext-cidr and ext-network to something other than the default before microstack.init is run.

In practice, however, this doesn't work. The values can be set ('sudo snap set microstack config.network.ext-network=<foo>'), but the .init script throws an exception.

This is probably because the default values have been written to templates. The .init script should rewrite the templates and restart services before it does anything else.

[Pen Gale (pengale)]

This issue causes a lockup when I try to test MicroStack using a MicroStack cloud. Because both the top level host and the MicroStack guests are attempting to use the same network, I lose connectivity to the guests. We need to avoid binding br-ex to the network until we've given an operator a chance to fix the network (or do some intelligent discovery and avoid clobbering an existing network on the host.)

[Pen Gale (pengale)]

https://review.opendev.org/#/c/693948/