example/ubuntu: add libevent network provider

- probe network by connecting to talk.google.com xmpp
- non blocking dns and connection
- replaces netlink based network

Bug: 24466635
Change-Id: I8a00731e18b8f155a8bf71f6c5593ca60d8612be
Reviewed-on: https://weave-review.googlesource.com/1303
Reviewed-by: Johan Euphrosine <proppy@google.com>