multiple ovirt repos should not be allowed at the same time

[landgraf]

Currently it's possible to specify multiple repos by using ovirt_rpm_repo, ovirt_repo_files and ovirt_repo_url variables at the same time.All repos will be installed. This can bring problems and dependencies hell.

I see few possible solutions (maybe there're more):

1. Check if two of them were specified and raise.
2. Use overriding (for example url overrides files and both of them overrides rpm).