Make it easier to use salt shaker for development

[mattmb]

Currently using salt shaker it is hard to work on the formulas locally and push them to a test environment.

Either:
In metadata.yml have a way to specify a local copy of the formula e.g.

- ../my-salt-formula
- ministryofjustice/python-formula

Or:
Have a command to run which "installs" the local version (probably by symlinking?) e.g. fab salt_shaker_install ../my-salt-formula

[SteveMarshall]

Have you seen how bundler handles this? That's a non-awful pattern to follow…