Reuse main LDAP connection
    
    cainstance and krainstance now reuse the main LDAP connection
    api.Backend.ldap2 in all helper functions. Some functions used to create
    and tear down their own LDAP connection. This was a remnant of the old
    CA LDAP instance in FreeIPA 3.x.
    
    Related: https://pagure.io/freeipa/issue/8521
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>