Update token seed improvements

[breml]

Additional improvements:

• Currently nothing prevents the user from changing a token seed definition while actual deployments are ongoing.
• While doing so does not really break ongoing deployments, since the seeded ISO are cached based on a fingerprint of the token seed values. Never the less, it would be good to have a way to notify the user in such a case and only allow updates e.g. with a force flag.
• Further more, if a user decides to change a token seed from public to private, the server should remove the respective cached images.
• The name of a token seed is not restricted about the characters that are safe to use. Since the name is later used as part of the URL to fetch the image, it would be wise to add some restrictions
• Also currently it is not possible to rename a token seed.