jsonrpc: client side heartbeat implementation

There is new vdsHeartBeat option in the database which default value is
10 seconds. This option is propagated down to json client.

Bug-Url: https://bugzilla.redhat.com/1115044
Change-Id: I9e8a20592542ce1a754dee1917ff9ebba6b8daf9
Signed-off-by: pkliczewski <piotr.kliczewski@gmail.com>