relating repo and installer via policy is wrong, should be repo => installer direct

At the moment the only relationship between repo and installer objects in Razor is through a policy. This is kind of crazy-indirect, since the installer and the repo content are pretty intimately related and, in general, the only reason to try and indirect their relationship would be if a repository could be used to install several different OS versions and those were implemented as multiple installation tasks.

We should fix this by:

1. Adding a relationship from repo to installer.
2. Removing the installer from policy.
3. Using a migration to move the data about.

lutter noted that users may want to have a custom installation task for, eg, advanced kickstart editing; I am uncertain this would ever coexist with the stock installer, or even if it did, that the space saving of having one repo instead of two is worth it.

We should decide if we want to support, eg, policy overriding repo for which installer to use before we finish this bug fix.