config consul source supports slash as prefix

`config.NewConfig()` with consul source will both read from consul
and watch consul for changes. Hence, the `prefix` is used in these
2 cases:

- read case: it is used to strip path based on the `KVPair` returned
from consul `kv.List()` method
- watch case: it is used as the `key` of watch query (`keyprefix` type)

So for *watch case*, the `key` is leagal to be `/` for watching change
on root. While for *read case*, because `KVPair.Key` is always stripped
off the leading slash, so if user specified some `prefix` with leading
slash, we should strip it also.

An extream case would be: user want's to read & watch node in root dir.
One would specify `prefix` as `/`, and it should work then.

config/source/consul/util.go

@@ -13,7 +13,7 @@ func makeMap(e encoder.Encoder, kv api.KVPairs, stripPrefix string) (map[string]
 
 	// consul guarantees lexicographic order, so no need to sort
 	for _, v := range kv {
-		pathString := strings.TrimPrefix(strings.TrimPrefix(v.Key, stripPrefix), "/")
+		pathString := strings.TrimPrefix(strings.TrimPrefix(v.Key, strings.TrimPrefix(stripPrefix, "/")), "/")
 		var val map[string]interface{}
 
 		// ensure a valid value is stored at this location