scripts: twister: decouple debug and verbosity

Currently, debug logging in the console and verbosity
are tightly coupled - verbosity of level 2 and higher
enables logging at the debug level.

This change introduces a separate Twister flag
responsible for controlling the debug logging,
while leaving the rest of verbosity unchanged.

This allows for controlling the verbosity on
both logging levels, according to one's needs.

Signed-off-by: Lukasz Mrugala <lukaszx.mrugala@intel.com>