Description of problem:
There is no way to define the default repository mirroring behavior.

Version-Release number of selected component (if applicable):
All versions of Satellite

How reproducible:
Always the case

Steps to Reproduce:
1. Create a custom product
2. Add a repository
3. Mirroring policy always defaults to "content only"

Actual results:
Default repository mirroring behavior is set to "content only"

Expected results:
This default behavior should be customizable.

Additional info:
In the customers environment, they have an https server that an offline Satellite mirrors content from. If the mirror policy is set to content only, then after the most recent mirror, there are only the NEWEST packages included in repository. All of the older packages are unavailable.

To avoid this, the customer must put the mirror policy into "additive" mode. If the customer forgets to do this step, it can cause them hours of corrective work. It would be much easier for them to create a default behavior for this.

Another alternative is to have the Satellite automatically set the default mirroring behavior to "Additive" when the Satellite is in disconnected mode.