migrations: ensure units have relation scopes in precheck

[babbageclunk]

Description of change

We were seeing CI migration tests fail because the migration was being
started before subordinate units had entered their relation's scope. Add
a precheck for this situation so that the user can see that they need to
wait for the units to finish coming up (rather than the migration
aborting for a cryptic reason).

QA steps

Bootstrap 2 controllers A and B.
Create model m in A.
Deploy ubuntu and ntp to m but don't relate them immediately.
Once the ubuntu unit is fully ready and idle, relate ntp and ubuntu, and immediately start repeatedly trying to migrate m to B.
You'll see a variety of pre-migration check errors, including:
ERROR source prechecks failed: unit ntp/0 hasn't joined relation ntp:ntp-peers yet
Eventually the unit will be ready and the migration will run successfully.

Bug reference

Example of failed test run:
http://qa.jujucharms.com/releases/5851/job/model-migration-lxd/attempt/2111

[babbageclunk]

This will mean that the test will still sometimes fail (although with the precheck error, rather than a generic "migraton aborted" error and a message about the relation scope in the logs). Once this is landed and has seen a test run I'll change the test to wait for the agents to be ready (it already waits for the application and workloads to be ready).

[babbageclunk]

$$merge$$

[jujubot]

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