This commit moves away from using a global variable for default
configuration checking, and instead exposes a method on LoadingRules to
determine whether a particular restclient.Config should be considered
"default". This allows kubectl to provide its own defaults (the same
as before, KUBERNETES_MASTER and the static localhost:8080 values) while
allowing other clients to avoid defining them.
In-cluster config defaulting is now easier to read.
07f079216e