Stop caching Whitehall organisations API

There is no need to cache the results of the organisations API, and
doing so makes it harder to reason about changes to organisation slugs.
This has caused confusion when:

- importing site config for new sites which belong to organisations
  which have been created in Whitehall on that day, and therefore
  don't yet exist in Transition because the API was cached before
  they had been created;
- changing the slug of existing organisations in Whitehall: the caching
  means that after the slug has been changed in Transition for the
  existing organisation, a subsequent run of import:orgs_sites_hosts
  later that day would create a new organisation for the old slug
  from the cached data.

The cached_orgs_path parameter remains so that we can still use test
fixtures to avoid making API calls in specs. We could alternatively
use the test helpers from gds-api-adapters instead of the existing
fixtures, but we would probably want to simplify our test data in
that case.
Rename cached_org_path to org_yaml_path

We aren't using this for caching any more, so this name is less
misleading. This also adds a comment to clarify that this parameter
is only for use in tests.
Organisation slug changer

- changes the whitehall_slug of the organisation itself
- changes the organisation_slug of any users which reference that
  organisation
Rake task to invoke slug changer
Merge pull request #462 from alphagov/rake-task-for-org-slug-change

Rake task for org slug change
Merge pull request #463 from alphagov/stop_caching_whitehall_orgs

Stop caching Whitehall organisations API