Fix logging of debug, info and warning levels.
Also configuration parameter VERBOSE is working again.
fixes problems:
* VERBOSE parameter can be changed at runtime
* Initial log level was unset, then there was nothing logged lower than
exception level. (for example configuration dump was not logged)
* Initial VERBOSE was set to True, then it was too much verbose. And
this flag was mean to be used for debugging problems.
NOTE: real log level is driven by collectd configuration, not by plugin
config.
Change-Id: Ia7048ccb74f27a5d5885b9c0bda17d6fba603e9b
Closes-Bug: #1664973
f903de3f10