confirm and document that bundles support charm_dir, repo_dir

People are writing this sort of thing in Mojo bundles:

  charm: {{ build_dir }}/../charms/{{ series }}/prometheus2

but BundlePhase is a subclass of DeployerPhase and therefore uses its template expansion logic, by calling self._get_configs in BundlePhase._get_phase_configs.

The above can thus be written

  charm: {{ repo_dir }}/{{ series }}/prometheus2

or even

  charm: {{ charm_dir }}/prometheus2

which are much nicer than the first example.

I suspect this came about because docs/mojo/readme.md documents the bundle phase as only supporting a subset of the template variables supported by the deploy phase.