[dns] launch weavedns by default

[rade]

Instead of a separate weave launch-dns step, we should launch weavedns by default, as part of weave launch. Unless the user specifies a --without-dns option.

This does depend on #22 somewhat, in that weavedns needs an IP (and subnet), and we don't really want to clutter weave launch with that information, at least not by default.

A few other points need consideration:

• if we pick the IP and subnet automatically, should it be possible to override that choice?
• should it still be possible to launch (and stop) weavedns separately?

We want to avoid option/command clutter, so unless we have compelling use cases for needing the above, we should refrain.

[rade]

#741, #762, #763 are all related.

[awh]

Fixed by #1009.