Minor Updates to Storage Policy Docs

Fixed some grammar issues, formatting issues, clarrified some
wording now that its easier to read fully rendered.

Change-Id: Ie803dd1a84d50de7663a7099c32d81807701e188
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
Don't count 412 or 416 as errors in stats

The backend HTTP servers emit StatsD metrics of the form
<server>.<method>.timing and <server>.<method>.errors.timing
(e.g. object-server.GET.timing). Whether something counts as an error
or not is based on the response's HTTP status code.

Prior to this commit, "success" was 2xx, 3xx, or 404, while everything
else was considered "error". This adds 412 and 416 to the "success"
set. Like 404, these status codes indicate that we got the request and
processed it without error, but the response was "no". They shouldn't
be in the same category as statuses like 507 that indicate something
stopped the request from being processed.

Change-Id: I5582a51cf6f64aa22c890da01aaaa077f3a54202
Merge "Replace POLICY and POLICY_INDEX with string literals"
Merge "Minor Updates to Storage Policy Docs"
Merge "Don't count 412 or 416 as errors in stats"