migration: Don't create invalid relation scopes for subordinates on import

[babbageclunk]

Description of change

Importing a model that had correct relation scopes for a subordinate with two principals would create invalid relaton scopes, which would cause problems if/when relations were removed after the import. (See bug below for more detail of problems.)

QA steps

• Deploy a subordinate related to two principals, eg:

ubuntu/0
  telegraf/0
mysql/0
  telegraf/1

• Removing and re-adding each relation (leaving time for the system to settle at each step) - the associated telegraf unit should go away and come back successfully.
• Migrate the model to another controller.
• Remove and re-add each relation as above - the telegraf units should be removed and recreated cleanly.

Bug reference

Fixes https://bugs.launchpad.net/juju/+bug/1715794

[babbageclunk]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://ci.jujucharms.com/job/github-merge-juju