Set log level at plugin init

[FlorentP42]

Is your feature request related to a problem? Please describe.
During testing or debugging, some logs that happen very early in the launch process could be missed if the lowest log level (debug) is not enabled from the start.

Describe the solution you'd like
Add a compile-time variable START_LOG_LEVEL that, if defined, will become the log level of this plugin at startup.
Example of usage when compiling for tests:

cmake -DCMAKE_BUILD_TYPE=Release -DFLEDGE_INCLUDE=/usr/local/fledge/include/ -DFLEDGE_LIB=/usr/local/fledge/lib/ -DCMAKE_CXX_FLAGS=-DSTART_LOG_LEVEL=debug ..

Describe alternatives you've considered
N/A

Additional context
Example of implementation in: fledge-power/fledge-south-hnz#77

[FlorentP42]

Actually this was a different topic than the erroneous debug level fixed by #79, reopening.