(PUP-1060) update yumhelper to handle {enable,disable}repo

In order to be able to use enablerepo and disablerepo flags while
ensuring latest, we need to be able to query for packages needing update
with a specific set of repos to enable and disable.

This commit updates yumhelper.py to take two flags '-e' for a repository
to enable, and '-d' for a repository to disable. The values given to
these flags must be a comma separated list of names with no spaces
between them.

It's unsustainable in the long run to continue extending this file, and
we should look very seriously at seeing what behaviors we can remove.
It's impossible to test this as-is and as we add more complexity we risk
introducing more bugs.

Original patch by Matthew Byng-Maddick.