Revert "Authenticate via oauth"

This reverts commit 149e1128818aacb24654f990430791562054fa93.

This commit I958983f505e261e3f1920a8df66eced3258b001e broke
stable/havana by introducing a dependency on oauthlib, but oauthlib
isn't in the stable/havana branch of global dependencies (fix in
progress Ib2e2aa2e75e7b7b43e0534eeb62e748d1edc0bc3). And we use trunk
clients in stable/havana gate jobs. So this patch broke both heat and
horizon, both of which import this file to support keystone's v3 API.

Change-Id: I56f7a8d2d2ddce0565f29bf29628555e4fe3c761