allow disabling of weaveDNS advertising/lookup

[rade]

...while keeping other weave integrations, by setting --weave.hostname=

Addresses a suggestion in #1550.

[rade]

There is a change in behaviour here, beyond the explicit intention of this PR, in what value we attempt to register/lookup in DNS. This is a) is unlikely to be relied on by existing users, and b) hard to eliminate:

weave.hostname	{app,probe}.weave.hostname	master	branch
unset	unset	scope.weave.local	scope.weave.local
unset	=		scope.weave.local
=	unset	scope.weave.local
unset	=y	y	y
=x	unset	x	x
=	=
=x	=	x	x
=	=y	y	y
=x	=y	x	y

[ekimekim]

LGTM. I agree it's very unlikely anyone was relying on that behaviour, and even if they are it's a simple fix.