GitLab

This happened due to config not cloning implicitly, then beeing
overwritten by sum_meetings and then not containing any URL at all
during the next iteration. Solved by creating a deepcopy

If the user provides a wrong parameter with --endpoint/-e
Instead of simply telling them they are wrong, list the available
endpoints instead.

Additionally display a warning if users are on server, so they know
their local config is ignored

Usage:
```
bbbmon -w 2
```

Note that this uses sleep and sleeps for e.g. 2 seconds inbetween
requests. A more accurate way would probably be to messure the time that
has passed since starting the request and subtracting that from the
interval.