Split kubespray-defaults (-> `network_facts`)

kubespray-defaults currently does two things:
- records a number of default variable values (in particular values used
  in several places)
- gather and compose some complex network facts (in particular,
  `fallback_ip` and `no_proxy`

There is no actual reason to couple those two things, and it makes using
defaults more difficult (because computing the network facts is somewhat
expensive, we don't want to do it willy-nilly)

Split the two and adjust import paths as needed.