debug: init debug and log level from environment

The environment variable WICKED_DEBUG=<mask> initialize the
debug mask and WICKED_LOG_LEVEL=<level> the level in similar
way as the --debug and --log-level option do.
For compatibility, the DEBUG variable is used if WICKED_DEBUG
is empty and is allowed to contain also the yes/no values.
Merge pull request #161 from mtomaschewski/debug

debug: init debug and log level from environment