core: jsonrpc only for 3.5 cluster compatibility level

When cluster level is set to lower level than 3.5 we must not let people
change default xmlrpc to jsonrpc. Lower versions of vdsm are not able to
understand jsonrpc so we disabled possibility on the UI.

Bug-Url: https://bugzilla.redhat.com/1142945
Change-Id: Ifaf0afa2abbd6ccc40a4fd84bebcb94544271639
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>