Enable persistent search by default
    
    From IPA version 3.0, the persistent search is a preferred mechanism
    to for DNS zone list management. It will be also a requirement for
    several bind-dyndb-ldap features, like SOA serial automatic updates
    or DNSSEC.
    
    Make this mechanism default in ipa-server-install and ipa-dns-istall.
    
    https://fedorahosted.org/freeipa/ticket/2524