Fixed unbounded local error issue

Proxy was never initialized in the else case.

After getting the latest clone, the following error occurs when issuing any API.

File "/home/andrew/error/jumpgate/jumpgate/identity/drivers/sl/tokens.py", line 170, in on_post
auth = identity.auth_driver().authenticate(credentials)
File "/home/andrew/error/jumpgate/jumpgate/identity/drivers/sl/tokens.py", line 116, in authenticate
proxy=proxy)
UnboundLocalError: local variable 'proxy' referenced before assignment
POST /v2.0/tokens 500 Internal Server Error [ReqId: req-070186de-0615-11e4-bf28-080027ba3401