Default xccdf_id arg to None for generate_fix_for_result function

[pep8speaks]

Hello @matusmarhefka! Thanks for submitting the PR.

• In the file openscap_daemon/oscap_helpers.py, following are the PEP8 issues :

Line 101:21: E128 continuation line under-indented for visual indent Line 121:80: E501 line too long (80 > 79 characters) Line 173:80: E501 line too long (80 > 79 characters) Line 201:80: E501 line too long (80 > 79 characters) Line 341:5: E722 do not use bare except' Line 371:80: E501 line too long (80 > 79 characters) Line 372:80: E501 line too long (80 > 79 characters) Line 538:1: E305 expected 2 blank lines after class or function definition, found 1