Allow for setting and cleaning of Marmotta home directory

* `marmotta.home` can be set in `jetty.yml` or using the `MARMOTTA_HOME`
environment variable. Using env variables, you can invoke Rake tasks like so:
`MARMOTTA_HOME=/home/me/marmotta rake jetty:start`
* The `jetty:clean` Rake task now empties out the Marmotta home directory.

Delivers [#83518656](https://www.pivotaltracker.com/story/show/83518656).