backend, frontend: warn users that host must be installed before upgrade

In backend we must check if nodeOS is null and in frontend
we should warn users that any node cannot be upgraded before
installed.

Change-Id: If8ad4f88a079c7f25b544320e10f1d2b5c4190b2
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1076221
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>