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)
Fix suggestions logic

This code was previously repeating the same action (setting the
suggestions key for the given package to list(provides) once for every
element in the set. While this did not produce a functionally incorrect
result, it is unnecessary and poorly-constructed as-is. This commit
simplifies the logic, and only performs that action once.
Remove unused variables
Merge pull request #11649 from terminalmage/yum-cleanup-2014.1

Fix suggestions logic (2014.1 branch)
Update test case to new return format.

Also, the comparison check is old, new or better, known -> unknown
Don't YAML'ify empty strings to `None`