Do not force people to use the swift-dispersion node

This role is optional. And people won't be able to assign more than one
node, thanks to the constraint (that is used for validation too).
Merge pull request #236 from vuntz/no-mandatory-dispersion

Do not force people to use the swift-dispersion node
Merge branch 'release/roxy/master' into roxy-sync-again
Merge pull request #237 from vuntz/roxy-sync-again

Merge roxy branch into stoney