Fix ASNUpdate test in rtarget_peer_test

Issue was that many of the config parameters got reset to default
values during the ASN change since the XML did not include older
values.  Fixed this by using a common config template for initial
and updated configuration.

Also add checks to make the test indirectly verify that BGP peers
get re-established.

Change-Id: Idb0a0d8fe91e8dbe25dbcacd3708832ce276e8af