Replace POLICY and POLICY_INDEX with string literals

Replaced throughout code base &  tox'd. Functional as well
as probe tests pass with and without policies defined.

POLICY --> 'X-Storage-Policy'
POLICY_INDEX --> 'X-Backend-Storage-Policy-Index'

Change-Id: Iea3d06de80210e9e504e296d4572583d7ffabeac
Merge "Replace POLICY and POLICY_INDEX with string literals"