Append trailing slash to folder listing in etcd3 backend

[benpaxton-hf]

Fixes an issue with etcd3 where listing keys in Vault can result in things that aren't children of the requested path being returned as if they were.

If you have a key secret/foo/bar, and a key secret/foobar/baz, and ask for a list of secret/foo, etcd3's non-hierarchical setup will return secret/foo/bar -> bar, and secret/foobar/baz -> bar/. Appending a slash causes secret/foobar/baz to no longer match, so the expected results are returned.

To show the issue;

vault write secret/foo/bar value=123
vault write secret/foobar/baz value=123

vault list secret/foo
vault list secret/fo

With etcd2:

$ vault list secret/foo
Keys
----
bar
$ vault list secret/fo
No value found at secret/fo/

With etcd3, before this PR:

$ vault list secret/foo
Keys
----
bar
bar/
$ vault list secret/fo
Keys
----
o/
obar/

With etcd3, after this PR:

$ vault list secret/foo
Keys
----
bar
$ vault list secret/fo/
No value found at secret/fo/

[jefferai]

@benpaxton-hf What version of Vault is the server? Modern Vault automatically adds slashes at the end of list operations within the HTTP API, so this shouldn't be necessary.

[benpaxton-hf]

Both etcd3 results are with `master` running on both client and server. `path.Join` strips trailing slashes, so even if there is a trailing slash in `prefix`, it's lost by using `path.Join`.

[jefferai]

Ah, that makes sense. Looks good, then.