webadmin: Add Ballooning to UI

This patch adds the necessary UI for enabling
and disabling ballooning feature on the cluster
level.

Change-Id: I8d8f0701f64a1ebf76965cfcbdec2912158c0c90
Bug-Url: https://bugzilla.redhat.com/967572
Signed-off-by: Ofri Masad <omasad@redhat.com>