Description of problem:
Currently, the user cannot remove an unregistered entity.
The only option is to register the entity to a data center and then remove it which is caused for redundant operations.
The problem is when we have a template with a disk that been copied to several storage domains for e.g on SD1 and SD2.
When detaching SD1 the template will be considered as an unregistered entity on
SD1 but it will also remain on the data center.
When the user will try to re-attach SD1 to that data center and register the template, he will fail because the template still exists.
So now the user has an unregistered template that he cannot remove or register.
[RFE] add an option to remove unregistered entities from attached storage domain
Version-Release number of selected component (if applicable):
Steps to Reproduce: