Fix handling of optional arguments after GLI conversion

Optional arguments where treated as required after the conversion to
GLI.
Allow optional arguments in method shift_arg
Fix handling of optional arguments after GLI conversion

Optional arguments where treated as required after the conversion to
GLI.
Separate assignment from check

This fixes the issue that the argument was always ignored because of
lazy checking.
Incorporate suggestions
Improve CLI error messages.
Merge pull request #13 from SUSE/fix_handling_of_optional_parameters

Fix handling of optional arguments after GLI conversion