Update LVM autoextend config for RHEL6

Cinder clones are slightly larger due to some extra metadata.  RHEL6
will not allow auto-extending of LV's without this, leading to clones
giving hard-to-track disk I/O errors.

See https://bugzilla.redhat.com/show_bug.cgi?id=975052

Change-Id: I09a5e061a9665c5310383f9f9eb281bfdc8e416d