fixes remove_instance error

* Initialize object attribute to None so a later if check that depends
on that attributes succeeds. The attribute is related to unifying the
command line args to the *_instance command set.
* Added regression test case

awx/main/management/commands/_base_instance.py

@@ -21,6 +21,7 @@ class BaseCommandInstance(BaseCommand):
 
     def __init__(self):
         super(BaseCommandInstance, self).__init__()
+        self.enforce_primary_role = False
         self.enforce_roles = False
         self.enforce_hostname_set = False
         self.enforce_unique_find = False