Retry http_request and json_request failure.

Temporary network outages or keystone server restarts can lead
to a connection refused in auth_token middleware. Rather than
failing immediately, this patch attempts to retry a few times.

Fixes bug 1150299

Change-Id: I2ecf0d7745290976efcb3e3cd6511817a53d3e0a
Merge "Retry http_request and json_request failure."