Quiet down Yum's logging. Fixes #3855.

Salt does not need any of `sys.stdout`, `sys.stderr` and `syslog` logging handlers which are configured by Yum, specially because we're using Yum as an API, not as another CLI tool. Any logging will go through salt's logging handlers.
Just set the levels as the Yum package does.
Merge pull request #8551 from s0undt3ch/hotfix/quiet-yum-logging

Quiet down Yum's logging. Fixes #3855.