Fixed tokens.Validate with GET Method instaed of HEAD

[cygmris]

Under stable/rocky of keystone
tokens.Validate with HEAD method won't works but GET

HEAD /v3/auth/tokens HTTP/1.1
Host: 127.0.0.1:5000
User-Agent: gophercloud/1.0.0
Connection: close
Accept: application/json
X-Subject-Token: gAAAAABbsJ2xgMz0O5ec33rym04sW1o-StKSOTOu7-jh03mevreKuYxwECuJ7l2CLCbE18HkRBjqIBtZz_pb981QuL3Sm30VtR7yPvcY6OYCQdoIxZxnKV0KrmzaQUke--PXPgqWxULQKDjha4CeFjDeY3keA9lh0A

HTTP/1.1 401 Unauthorized
Date: Sun, 30 Sep 2018 09:56:01 GMT
Server: Apache/2.4.34 (Ubuntu)
WWW-Authenticate: Keystone uri="http://127.0.0.1:5000"
Content-Length: 114
Vary: X-Auth-Token
X-Distribution: Ubuntu
x-openstack-request-id: req-f04eabfb-94f3-4740-958a-dbfa330f5234
Connection: close
Content-Type: application/json