Make part of the Subscribers object modeled

Make part of the Subscribers object modeled

This is a first step in solving bug 7272 and propagating subscriber data
to slave nodes in a cluster. This patch makes the YANG additions,
renames the SubscriberRLOC class to Subscriber, which makes more sense
(originally the SubscriberRLOC object only held the RLOC, now it holds
the source EID too), and uses the modeled SubscriberAddresses inside
Subscriber to store the source RLOC and EID.

Change-Id: I2e3ae71e7c9948d962921553b00cfecd76f445f1
Signed-off-by: Lorand Jakab <lojakab@cisco.com>