Support client generate literal ipv6 auth_uri base on auth_host

This patch is for fix bug 1208784. Support keystone client generate
literal ipv6 auth_uri via provided auth_host, when the auth_uri is
None.

Fixes bug 1208784

Change-Id: I226881a35a74ef668d4cd1c6829a64c94ff185d9
Replace HttpConnection in auth_token with Requests

Requests is becoming the standard way of doing http communication, it
also vastly simplifies adding other authentication mechanisms. Use it in
the auth_token middleware.

This adds the ability to specify a CA file that will be used to verify a
HTTPS connections or insecure to specifically ignore HTTPS validation.

SecurityImpact
DocImpact
Partial-Bug: #1188189
Change-Id: Iae94329e7abd105bf95224d28f39f4b746b9eb70
Merge "Support client generate literal ipv6 auth_uri base on auth_host"
Merge "Replace HttpConnection in auth_token with Requests"