Description of problem:
Once the Release Version and Service Level fields have been saved a value they cannot change back to a 'blank' value. Meaning, in practice, that an activation key for systems meant for premium subscriptions will never be able to change back to the default 'blank' value that will allow the assignment of any service level subscription available on the key to use.

How reproducible:
100%

Steps to Reproduce:
1. Create activation key
2. Set service level and save
3. Attempt to change service level back to 'blank'

Additional info:
The release version does not cause any immediate problems, it seems. Using a 7Server release version key still works for a rhel 6 system. The system is only given access to rhel 6 repositories (with rhel 6 and 7 being available from the key). Everything seems okay here, only that the rhel 6 system is given a release version of 7Server.