Fix broken el7 mocks, remove legacy mock config

This PR fixes some breakages in the el7 mock config by removing mirrorlists and proxies and also removes the need for the legacy mock definitions by rolling the logic into the main template.
Do we want to just turn this into:
```
config_opts['chroot_setup_cmd'] = 'install @buildsys-build'
```
Doesn't fix anything; just for consistency.