Abstract checking own address into a function

Currently, we check if we have a given ip address (i.e. it's alive on
one of our interfaces) but manually calling TcpPing(source=localhost).
This works, but having it spread all over the code makes it hard to
change the implementation.

The patch abstracts this into a separate utils.OwnIpAddress(addr)
function. We add a rpc call for it, which we use instead of the
(single-use of) call_node_tcp_ping. We leave node_tcp_ping in, as seems
useful and eventually it should be removed in a separate patch.

Reviewed-by: imsnah