separate function to set ipaConfigString values on service entry
    
    There is some code duplication regarding setting ipaConfigString values
    when:
       * LDAP-enabling a service entry
       * advertising enabled KDCProxy in LDAP
    
    We can delegate the common work to a single re-usable function and thus
    expose it to future use-cases (like PKINIT advertising).
    
    https://pagure.io/freeipa/issue/6830
    
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>
    Reviewed-By: Simo Sorce <ssorce@redhat.com>