webadmin: check UP server before adding bricks

While adding the bricks after creating a volume, Add Bricks
dialog will be shown only if the cluster has a host in UP status.
Otherwise an error message denoting no up server found in the cluster
will be shown.

Change-Id: I54b3c0eacbe4c94929d20738196324569ed8f3b6
Bug-Url: https://bugzilla.redhat.com/907338
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>