Refactor storage encryption to support multi-pod deploy

As we are moving towards a multi-pod deployment in Openshift, the storage encryption mechanism should also be adjusted to support that.

Together with that change, a review could be done also over the tvb-storage API methods that are specialized for storage encryption, and assess whether these calls could be hidden by the tvb-storage.
Ideally, the storage encryption handling should be hidden under the tvb-storage and no specialized method would be exposed in the StorageInterface.
Eg for controllers that need to notify the tvb-storage when a certain project should be decrypted. We could hide that under a generic method like StorageInterface.set_project_active().