replica-prepare: do not add PTR records if there is no IPA managed reverse zone
ipa-replica-prepare could crash during addition of replica's PTR records if
there was no reverse zone managed by IPA and 'bindinstance.find_reverse_zone'
returns an unhandled None. The code will now issue a warning and skip the PTR
record addition in this case.
https://fedorahosted.org/freeipa/ticket/5740
Reviewed-By: Martin Basti <mbasti@redhat.com>