`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.
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