CustodiaClient: use ldapi when ldap_uri not specified
Reinstate ldap_uri selection behaviour that was lost during the
refactor in the previous commit. This is required because, on the
ipa-4-7 branch at least, the IPASecStore needs to use LDAPI to set
the Directory Manager passphrase digest. Use LDAPI when the
ldap_uri has not been explicity specified.
Part of: https://pagure.io/freeipa/issue/7964
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>