Config restructure

Simplify charm configuration as follows:
- Remove `code_archive_uri` option
- Make `code_archive` option take either a URL or a path within the charm
- Make `app_label` default to unit name, so it doesn't need to be set
- Create `code_archive`, `wsgi_application`, `apt_dependencies` and `nagios_index_content` config options so that the basic features of the charm are usable without editing `playbook.yml`