Update database fields

[NicoLaval]

You provide configure.py command to set default slug and backend URL (and some other minor things about other datasets) but I didn't find a way to update directly and simply other fields.

I would like to set the mapViewBaseURL variable.

I provide a PR (see #111) to do so, adding update.py command to update every variable in database with:

python manage.py update default mapViewBaseURL "${MY_PETRIMAPS_URL}"