plugins: set upstream unconditionally (#2956)

`upstream` is not needed as a setting; just set if unconditionally and
remove all documentation and tests for it.

At some point we want remove the hanlding for `upstream` as well and
error out on seeing it.

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/auto/setup.go

@@ -100,6 +100,7 @@ func autoParse(c *caddy.Controller) (Auto, error) {
 		for i := range a.Zones.origins {
 			a.Zones.origins[i] = plugin.Host(a.Zones.origins[i]).Normalize()
 		}
+		a.loader.upstream = upstream.New()
 
 		for c.NextBlock() {
 			switch c.Val() {
@@ -148,8 +149,8 @@ func autoParse(c *caddy.Controller) (Auto, error) {
 				a.loader.ReloadInterval = d
 
 			case "upstream":
+				// remove soon
 				c.RemainingArgs() // eat remaining args
-				a.loader.upstream = upstream.New()
 
 			case "transfer":
 				t, _, e := parse.Transfer(c, false)