BUG 1699941 stop ignoring region-config values (backport to 2.2)

[hmlanigan]

Description of change

Add the region-config values for use during bootstrap. They should over
write the cloud-config values, but not any config values specified on the CLI
during bootstrap.

This is a straight backport of #7634 to the 2.2 branch.

QA steps

1. Create a yaml cloud config file to add an openstack cloud, including both cloud-config and region-config values. Suggest using the network config attribute, where only the related value in region-config will work.
2. Add an openstack cloud to juju with yaml file from step two.
3. Bootstrap the new cloud.
4. Try to bootstrap the new cloud including --config network=no-such-network and watch it fail.

Bug reference

https://bugs.launchpad.net/juju/+bug/1699941

[hmlanigan]

No review needed, trivial backport.
$$merge$$

[jujubot]

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