Use Guzzle 4 as the transport layer

Guzzle 4 + Guzzle streams uses the same interface as described in the PHP FIG proposed http messages recommendation at https://github.com/php-fig/fig-standards/blob/master/proposed/http-message.md. I asked in the FIG IRC room and was told if I'm going to need an interface I should be ok using this one. Just to check back later and see if anything changed. Proceeding based on this interface.

Gerrit topic: https://review.openstack.org/#q,topic:bp/guzzle-transport,n,z

Addressed by: https://review.openstack.org/86137
    Migrating the transport layer to use the proposed FIG PSR for http messages and to use Guzzle 4 as the default transport layer.

Gerrit topic: https://review.openstack.org/#q,topic:bug/1277535,n,z