engine: Added query for external networks on provider

This query allows to discover all the networks on the given
provider and returns as a result the list of networks which can
then be used to, for example, add the network to the system.

Change-Id: I2954addacfea09e0b44bf2770f80be92f75b6db5
Signed-off-by: Mike Kolesnik <mkolesni@redhat.com>