Prevent error from being raised when user is not trying to update a

disabled env role

We were only checking to see if a role was disabled or deleted before
raising an error, so I added in a check to see if the user was trying to
update the env role before raising an error. The error should only be
raised if the role is disabled or deleted AND the user is trying to
assign a new role to the env role.

I also added in a disabled property to the EnvironmentRole model to make
things more readable.

atst/models/environment_role.py

@@ -66,6 +66,10 @@ class EnvironmentRole(
     def displayname(self):
         return self.role
 
+    @property
+    def disabled(self):
+        return self.status == EnvironmentRole.Status.DISABLED
+
     @property
     def event_details(self):
         return {