Avoid failing at import time if krbV module is missing

Several parts of Koji already handled this absence gracefully. This change
extends that behavior to all places where krbV is imported.

builder/kojid

@@ -24,7 +24,7 @@
 try:
     import krbV
 except ImportError:  # pragma: no cover
-    pass
+    krbV = None
 import koji
 import koji.plugin
 import koji.util
@@ -5782,7 +5782,7 @@ if __name__ == "__main__":
             quit("Error: Unable to log in. Bad credentials?")
         except xmlrpclib.ProtocolError:
             quit("Error: Unable to connect to server %s" % (options.server))
-    elif 'krbV' in sys.modules:
+    elif krbV and 'krbV' in sys.modules:
         krb_principal = options.krb_principal
         if krb_principal is None:
             krb_principal = options.host_principal_format % socket.getfqdn()