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/consul.go

@@ -21,7 +21,7 @@ type consul struct {
 var (
 	// DefaultPrefix is the prefix that consul keys will be assumed to have if you
 	// haven't specified one
-	DefaultPrefix = "micro/config/"
+	DefaultPrefix = "/micro/config/"
 )
 
 func (c *consul) Read() (*source.ChangeSet, error) {