1098891: Update repos, persisting local settings when possible

# Preserve Existing Repos Settings

## The Problem: [BZ 1098891](https://bugzilla.redhat.com/show_bug.cgi?id=1098891)

People like to enable/disable yum repositories with yum-config-manager, which essentially updates the redhat.repo file:
```
yum-config-manager --enable rhel-7-for-power-supplementary-source-rpms
```
However, we do not allow manual edits of our repo file, so when any yum command is run, we reset the **enabled** flag to default.


## Solution

Allow old style repository updates (currently only used for disconnected systems).  The drawback is that changing default values of mutable fields will **not result in changes to the redhat.repo file**

Expected changes should be created as repo-overrides, which will always override existing values