server-install: No double Kerberos install

When we're installing server with an external CA, the installation
would have failed in the second step where it's passed the required
CA cert file because it would have tried to perform the Kerberos
installation for the second time.

https://pagure.io/freeipa/issue/6757

Reviewed-By: Jan Cholasta <jcholast@redhat.com>

ipaserver/install/server/install.py

@@ -762,11 +762,12 @@ def install(installer):
             options.subject_base, options.ca_subject, 1101, 1100, None)
 
     krb = krbinstance.KrbInstance(fstore)
-    krb.create_instance(realm_name, host_name, domain_name,
-                        dm_password, master_password,
-                        setup_pkinit=not options.no_pkinit,
-                        pkcs12_info=pkinit_pkcs12_info,
-                        subject_base=options.subject_base)
+    if not options.external_cert_files:
+        krb.create_instance(realm_name, host_name, domain_name,
+                            dm_password, master_password,
+                            setup_pkinit=not options.no_pkinit,
+                            pkcs12_info=pkinit_pkcs12_info,
+                            subject_base=options.subject_base)
 
     if setup_ca:
         if not options.external_cert_files and options.external_ca: