Use X-Backend-Storage-Policy-Index

The reconciler adds a number of X-Backend headers to container
responses and also override InternalClient requests which over-ride
container_info when setting X-Backend-Storage-Policy-Index.  Since
only backend requests will ever refer to policy by index, this change
unifies this backend request header with the other headers already
prefixed by Backend.  It also superficially simplifies the proxy's
BaseController translation of policy_index to name since X-Backend
headers are already stripped from outgoing requests.

Change-Id: I7597dac2bde7fc438c96b20e1574508aba5cfa05
Merge "Use X-Backend-Storage-Policy-Index" into feature/ec