Avoid updating fields when uneccessary

When filling lists of hosts the change event is raised. To avoid
updating fields when we don't need it, do this update only in the right
state - if discovered host form is enabled for example, this to avoid
overriding the fields while fetching hosts info from foreman.

Change-Id: I7de552c03e361b5186c6781eabd6dc4da34f9fb5
Signed-off-by: Yaniv Bronhaim <ybronhei@redhat.com>