Optionally load advanced configuration file from `/etc/aura/steering.py`

To avoid having complicated instruction to load advanced settings, like for LDAP configuration, we want to optionally load the file located in /etc/aura/steering.py. This should be possible with Django's execfile().

Note, this extra settings file should be optional. Meaning it can be mounted into the Steering Docker container, but it's not guaranteed. Therefore the original settings file needs to have a check if a.) the path is existing and b.) if it's not a folder. Compare the implementation in other AURA repositories.