Attempting to change scope type of existing scoped role leads to error on save. 

Steps to Reproduce:
1. navigate to Administration - Role Assignment - Roles - Scoped Roles
2. create role of group scoped role restricted to main-server-group
3. select the role created in previous step in the list and click "edit" button
4. change type to "Host" and select the scope (e.g. master host)
5. save 

Actual results:
"Failed to save error message"

Expected results:
As both "Host" and "Server group" scoped roles are a different type of resource, it should not be possible to modify the scope type of existing roles.

Proposed fix:
Remove the option to change scope type