Report migration status in show-model

[babbageclunk]

Adds migration, migration-start and migration-end (for failed migrations) fields to the status section of show-model output. These display details about the current migration, if one is running, or the latest failed migration - if the last migration was completed successfully the model would be gone.

Fixes https://pad.lv/1569632

Example output:

my-model:
  name: my-model
  model-uuid: 7a6051e0-ba0b-4b35-8d9b-627072092933
  controller-uuid: fb82ce82-6da4-4db3-8dc3-01c28f2f25bc
  controller-name: source
  owner: admin
  cloud: lxd
  region: localhost
  type: lxd
  life: alive
  status:
    current: available
    since: 5 minutes ago
    migration: validating, waiting for agents to report back
    migration-start: 10 seconds ago
  users:
    admin:
      display-name: admin
      access: admin
      last-connection: never connected

Testing done:

• bootstrapped 2 controllers with the migration feature flag on
• (in source controller) add-model my-model
• deploy ubuntu
• migrate my-model dest
• watch -n 0.5 juju show-model

[babbageclunk]

!!retry!!

[babbageclunk]

This is weird - it's linking to the build for #6543 (http://juju-ci.vapour.ws/job/github-check-merge-juju/176/console) which I incorrectly created against the staging branch. Not sure how to get it to run again for this PR.

[babbageclunk]

!!build!!

[babbageclunk]

!!build!!

[babbageclunk]

$$merge$$

[jujubot]

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

[jujubot]

Build failed: Building binary
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/9627

[babbageclunk]

It ran out of memory while building the binary. :/
$$retry$$

[jujubot]

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

[jujubot]

Build failed: Building binary
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/9628

[babbageclunk]

@Veebers kicked some things for me.
$$retry$$

[jujubot]

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

[jujubot]

Build failed: Tests failed
build url: http://juju-ci.vapour.ws:8080/job/github-merge-juju/9629

[babbageclunk]

Looks like mongo crapped itself on Windows.
$$retry$$

[jujubot]

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