Quantum metadata handler now uses X-Forwarded-For

fixes bug 1097524

The guest IP address was not properly forwarded from the Quantum
metadata proxy.  This patch adds support in Nova for reading this
X-Forwarded-For header.  Previously, the internal proxy IP was
mistakenly used exposing internal infrastructure details.

Change-Id: I6d7d21e2aea4ca5debf1ca4ba6802012537f78a5
Merge "Quantum metadata handler now uses X-Forwarded-For"