Ignore retcode for yum refresh_db

'yum -q check_update' returns a retcode of 100 if there are updates to
install, so ignore the retcode and don't log an error if a nonzero
retcode is seen here.
Merge pull request #11646 from terminalmage/yum-refresh-ignore-retcode-2014.1

Ignore retcode for yum refresh_db (2014.1 branch)