'weave launch' without CIDR

It turns out we don't need a CIDR in `weave launch`.
The `ethwe` interface in the `weave` container does not an IP address/netmask since we only use it to perform captures and injects. So the only place `weave launch` really needed the CIDR was for setting up masquerading. But that is just an enabler for `weave expose`, so it's actually more logical to configure masquerading there, and only for the specific subnet exposed.

For backward compatibility, now `weave launch` swallows any supplied CIDR and shows a warning when it does so.

Closes #130