Omero home web settings 11373

This removes the use of ``` os.environ.get('OMERO_HOME') ``` from omeroweb/settings.py. This means you can run OMERO web in production OR development locations without setting OMERO_HOME.

To test (need dist/lib/python on your $PYTHONPATH):
```
# run from production
$ bin/omero web start
```
Check that web is working OK, E.g. logging & static files etc
```
# run from development (remove dist libs)
$ rm -rf dist/lib/python/omeroweb
$ cd components/tools/OmeroWeb/omeroweb
$ python manage.py runserver
```
Again, check that web is working OK, E.g. logging & static files etc