mgr/dashboard: improve cluster log wording and optionally show debug logs
Currently the cluster > logs page only shows loglevel INFO and above. While this is completely sensible, sometimes that leads user to not see any log trace (Blank page with "No entries found" message), which makes them suspicious of a failure ("found/not found") or an improper reporting (this is partly due to misconceptions on what the cluster/mon log is vs. daemon logs).
An alternative would be to display a more explanatory message ("No log entries with level INFO, WARN or ERR. DEBUG logs are not displayed").
Additionally we could also allow for DEBUG messages to optionally be displayed ("Click here to show also DEBUG messages"). Nevertheless, DEBUG messages would be a repetition of the followint pattern:
2019-11-22 12:40:43.019173 mgr.<host> (mgr.xxxx) xxxx : cluster [DBG] pgmap vxxxx: 733 pgs: 733 active+clean; 1.1 TiB data, 1.7 TiB used, 6.5 TiB / 8.2 TiB avail; 3.8 KiB/s rd, 3 op/s