Allow skydns itself to use env variables

Reuse the variables defined in skydnsctl for skydns itself. Also rename
them slightly to keep the two much more in sync.

SKYDNS_DNSPORT -> SKYDNS_DNS (something like 127.0.0.1:1053) in stead of a number
SKYDNS_DNSDOMAIN -> SKYDNS_DOMAIN (a string like skydns.local)

The environment variable SKYDNS is used in skydns, the prefix http:// is
removed before use. This might confuse people.

Sadly this is a minor backward incompatible change, because
skydns.NewClient needed to be changed