Improvements for dry-run cli

From the comments on #52 and #48:

- We could avoid having to specify the Galaxy job conf by reading `galaxy.yml` and parsing the job conf out of there. However, adding this functionality directly to TPV (including automatically locating `galaxy.yml`) is reinventing a wheel (and a rather complicated and messy one) that is already done by Gravity and Galaxy itself. Ideally we would separate this out into a library that all 3 projects could then use.
- The dry-run is not going to work for more complex custom rules because we are mocking Galaxy objects that won't have all of the attributes that a custom rule might access (e.g. `object_store`). We can look at a couple of things, including Galaxy's IntegrationTestCase and the minimal app that celery workers load.
- There is currently no way to specify any tool param values ~~or input sizes~~ when performing a dry run. EDIT: `--input-size` added to #52.