Support Vault cluster installations

[VladRassokhin]

It's actually cause of #4.
Sometimes when we trying to get wrapped token server responds with

HTTP/1.1 307 Temporary Redirect

Cache-Control: no-store
Location: https://vault-node-2.internal:8200/v1/auth/approle/login
Date: Mon, 27 Nov 2017 15:16:51 GMT
Content-Length: 0
Content-Type: text/plain; charset=utf-8