Make yumpkg module log locally to /var/log/yum.log

The addition of _YumErrorLogger effectively killed local yum logging.
This commit modifies the logger to behave more like
yum.rpmtrans.RPMBaseCallback, bringing back logging to /var/log/yum.log
while preserving the special error logging that this class initially was
written to provide.
Minor logging tweak

This just tweaks the order of when the minion logs certain things, for
consistency.
Add notes on yum.conf and syslog

Fixing the logging issues in 3b8b6de uncovered some interesting info
about how yum handles syslog. This commit adds that to the top docstring
for the yumpkg module, as it may prove useful to users.
Merge pull request #5798 from terminalmage/issue5569

Make yumpkg module log locally to /var/log/yum.log