Improve database backend

[egede]

The database backend for Ganga using a MongoDB has some performance problems that are limiting its use.

• The startup time of Ganga can be very long
• If there is a failure in the container (running the database) starting, the error messages are not forwarded and it can be very hard to tell what is going on).
• The disk space taken up by the
• The error messages if docker is not available are unclear.
• The config object has wrong upper/lower case compared to instructions. Help text says Singularity but in reality it should be singularity.
• The large size of the journal files is causing trouble on shared file systems (due to size, but more due to slow write speed).