Fix NVP plugin to send notifications for gateway-less subnets

It was noted that an update notification should be sent
regardless; this patch addresses that. Since there is
no longer the need to distinguish on whether to send
the RPC message or not, the operation has been factored
out to avoid code duplication.

Closes-Bug: 1220881

Change-Id: If553a84b7221f8c98d758654d317217a909c43dc
Merge "Fix NVP plugin to send notifications for gateway-less subnets"