Check for CA subject name collision before attempting creation

Lightweight CA subject name collisions are prevented by Dogtag
(response code 409 Conflict), however, we do not want to expose the
Dogtag error.  Perform the check in the IPA framework as well,
raising DuplicateEntry on collision.

Fixes: https://fedorahosted.org/freeipa/ticket/5981
Reviewed-By: Milan Kubik <mkubik@redhat.com>

ipaserver/plugins/ca.py

@@ -163,6 +163,13 @@ class ca_add(LDAPCreate):
         except errors.NotFound:
             pass
 
+        # check for subject collision before creating CA in Dogtag
+        result = api.Command.ca_find(ipacasubjectdn=options['ipacasubjectdn'])
+        if result['count'] > 0:
+            raise errors.DuplicateEntry(message=_(
+                "Subject DN is already used by CA '%s'"
+                ) % result['result'][0]['cn'][0])
+
         # Create the CA in Dogtag.
         with self.api.Backend.ra_lightweight_ca as ca_api:
             resp = ca_api.create_ca(options['ipacasubjectdn'])