fix: improve oval v2 parsing to prevent unnecessary exceptions

[westonsteimel]

Previously this would log an exception stacktrace for parsing any criteria or criterion element that didn't have a comment attribute

[INFO ] downloading OVAL file for SLES 12 from https://ftp.suse.com/pub/projects/security/oval/suse.linux.enterprise.server.12.xml.gz
[DEBUG] http GET https://ftp.suse.com/pub/projects/security/oval/suse.linux.enterprise.server.12.xml.gz
[INFO ] parsing ./data/sles/input/source/oval/suse-linux-enterprise-server-12.xml.gz
[ERROR] returning results early due to exception in _parse_sub_group
Traceback (most recent call last):
  File "/Users/weston/github/anchore/vunnel/src/vunnel/utils/oval_v2.py", line 190, in _parse_group
    test_ids = VulnerabilityParser._parse_sub_group(criteria_element[1], config, config.artifact_regex)
  File "/Users/weston/github/anchore/vunnel/src/vunnel/utils/oval_v2.py", line 231, in _parse_sub_group
    regex_match = re.search(regex, criterion.attrib["comment"])
KeyError: 'comment'
[ERROR] returning results early due to exception in _parse_sub_group
Traceback (most recent call last):
  File "/Users/weston/github/anchore/vunnel/src/vunnel/utils/oval_v2.py", line 190, in _parse_group
    test_ids = VulnerabilityParser._parse_sub_group(criteria_element[1], config, config.artifact_regex)
  File "/Users/weston/github/anchore/vunnel/src/vunnel/utils/oval_v2.py", line 231, in _parse_sub_group
    regex_match = re.search(regex, criterion.attrib["comment"])
KeyError: 'comment'
[ERROR] returning results early due to exception in _parse_sub_group
Traceback (most recent call last):
  File "/Users/weston/github/anchore/vunnel/src/vunnel/utils/oval_v2.py", line 190, in _parse_group
    test_ids = VulnerabilityParser._parse_sub_group(criteria_element[1], config, config.artifact_regex)
  File "/Users/weston/github/anchore/vunnel/src/vunnel/utils/oval_v2.py", line 231, in _parse_sub_group
    regex_match = re.search(regex, criterion.attrib["comment"])
KeyError: 'comment'
[ERROR] returning results early due to exception in _parse_sub_group
Traceback (most recent call last):
  File "/Users/weston/github/anchore/vunnel/src/vunnel/utils/oval_v2.py", line 190, in _parse_group
    test_ids = VulnerabilityParser._parse_sub_group(criteria_element[1], config, config.artifact_regex)
  File "/Users/weston/github/anchore/vunnel/src/vunnel/utils/oval_v2.py", line 231, in _parse_sub_group
    regex_match = re.search(regex, criterion.attrib["comment"])
KeyError: 'comment'
[ERROR] returning results early due to exception in _parse_sub_group
Traceback (most recent call last):
  File "/Users/weston/github/anchore/vunnel/src/vunnel/utils/oval_v2.py", line 190, in _parse_group
    test_ids = VulnerabilityParser._parse_sub_group(criteria_element[1], config, config.artifact_regex)
  File "/Users/weston/github/anchore/vunnel/src/vunnel/utils/oval_v2.py", line 231, in _parse_sub_group
    regex_match = re.search(regex, criterion.attrib["comment"])
KeyError: 'comment'
[ERROR] returning results early due to exception in _parse_sub_group