restapi: Attaching network to cluster failure

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

Based on commit 0888726b8e5c39cacaefd5af9242513507d86292
which was reverted.

Change-Id: Ia43074bc0819b19873e11ac0203cd518459e6644
Bug-Url: https://bugzilla.redhat.com/961046
Signed-off-by: Moti Asayag <masayag@redhat.com>