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.

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]

Oops - still getting used to the process. Closing this and recreating against develop.