ci: Support for negative pylint rating

ci: Support for negative pylint rating

Script ./check was updated to correctly parse and process negative pylint
ratings. Also awk expression was fixed to get rid of error messages
during parsing of pylint output.

JIRA: VSPERF-587

Change-Id: I72c5bd85f38274f308fce881517e5b6fa4a123dc
Signed-off-by: Martin Klozik <martin.klozik@tieto.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Christian Trautman <ctrautma@redhat.com>
Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>
Reviewed-by: Richard Elias <richard.elias@tieto.com>