stbt.py: 'verbose' value not read from config file

'_debug_level' is supposed to get the value of 'verbose' parameter from
the stbt config file if none of '-v' or '--verbose' command line arguments
are specified.

This parameter didn't have any effect because the 'default=...' argument
of 'parser.add_argument' only updates the internal dictionary of
ArgumentParser but doesn't set '_debug_level'; and 'IncreaseDebugLevel'
is only called if ArgumentParser reads any of '-v' or '--verbose'
command line arguments.

This correction sets '_debug_level' to the value of 'verbose' and then
overwrites it when IncreaseDebugLevel is called.