bugfix #490 and added possibility of silent installation using database-urls

**Fix for #490**
While trying to deploy modoboa, it throws an error due to a wrong path. This is most likely related to the following commits:

https://github.com/tonioo/modoboa/commit/02b1388d795d2b62360b3acaf1ae3d165a9c6514 and / or https://github.com/tonioo/modoboa/commit/937e09c7235f64f041f595b17687193c02782493

The new template path was also missing in the MANIFEST.in
`IOError: [Errno 2] No such file or directory: '/Users/simme/.virtualenvs/test/lib/python2.7/site-packages/modoboa-1.0.1-py2.7.egg/modoboa/core/commands/commands/templates/settings.py.tpl'`

**Implementation of #489**
In order to make the instalation using salt stack, modoboa-admin.py needs to have extra parameters for database configuration

If a users enters a nonsense url like "postgres://" it falls back to asking questions...

**please see:**
http://dev.modoboa.org/issues/490
and
http://dev.modoboa.org/issues/489