certupdate: only add LWCA tracking requests on CA servers
ipa-certupdate throws an exception when executed on a non-CA server
in a CA-ful deployment with lightweight sub-CAs (LWCAs). Check that
we are on a CA server before attempting to create Certmonger
tracking requests for LWCAs.
HOW TO TEST
1. Install first server (with CA)
2. Install replica without CA
3. Create sub-CA (`ipa ca-add`)
4. Run `ipa-certupdate` on replica. Observe that no stack trace is
produced.
Fixes: https://pagure.io/freeipa/issue/8399
Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>