Fix setting gateways

There's a number of conditions in setting gateways that don't work
properly. Specifically, one of the issues is setting a gateway on an
address that already exists. It returns

Fix setting gateways

There's a number of conditions in setting gateways that don't work
properly. Specifically, one of the issues is setting a gateway on an
address that already exists. It returns a PropertyValueConflict error
on Ipv4Addresses/1/Gateway with Ipv4Addresses/1/Gateway

Obviously an address can't conflict with itself, so this is wrong.

To address this, move the gateway setting and selection code into a
routine outside of the main loop, after all the gateways are accounted
for, and so we can treat them separately.

Tested;
PATCH to an existing ip address works, and no longer returns the error.

More test cases likely needed.

Change-Id: I0339e02fc27164337416637153d0b0f744b64ad8
Signed-off-by: Ed Tanous <etanous@nvidia.com>

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