webadmin: Adds warning when upgrading cluster level

Adds warning when upgrading cluster level which contains some non
responsive hosts, that these hosts may become non operational after
connectivity is restored.

Change-Id: I88f47dd84c030bcd723f41c7973c4dae32df9f2e
Bug-Url: https://bugzilla.redhat.com/953532
Signed-off-by: Martin Perina <mperina@redhat.com>