prefix in consul api starts with no leading slash

When `consul.StripPrefix(true)` is set, current impl. will pass the
specified prefix (or default prefix) when calling consul api.

However, `prefix` in consul api starts with no leading slash, so
the default prefix (`/micro/config`) doesn't actually work.

I avoid code changes (esp. the one in `util.go`) to eliminate
impact on users who already notice it.

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) {