Now it's off by default, and there is an option to enable the
multicast route per-network.
We turn it on for the `weave` network created when we start the plugin.
Note that we remove the ability to set the weavemesh network name, or
disabling it altogether. This was always poorly support, since the
removal operation only ever operated on the default name. The option
was also not listed in the usage, though it was in the docs.
Matthias Radestock