engine: Use VdsDao for fetching hosts in NetworkClusterHelper

Instead of using the search engine for fetching host
internally, a direct use of the VdsDao is done.

In addition, skip DB call for the hosts if the network
is not required.

Change-Id: Icf6574700974b800d96627da9439d7793b7d31b2
Signed-off-by: Moti Asayag <masayag@redhat.com>