restapi : AttachNetworkToVdsGroupCommand HTTP Status 500 - java.lang.NullPointerException(#961046)

The server throws an internal server error when
add network command is executed with invalid network
name

Change-Id: Ie0102d1d36de7012ebd98c53ef8eb106ac7b04b7
Bug-Url: https://bugzilla.redhat.com/961046
Signed-off-by: Ravi Nori <rnori@redhat.com>