Use HTTP response codes for identity v3

The HTTP 1.1 standard (http://tools.ietf.org/html/rfc2616)
defines the "reason phrase" for each status code. For example,
400 is Bad Request, 404 is Not Found, etc.

The identity V3 API reference had unexpected values listed for
the return codes for different requests, such as
"identityFault" and "overLimit".

There's no HTTP response code for identityFault, so this was
removed.

The others were changed to use the standard reason phrase.

Closes-Bug: #1338128

Change-Id: I5ea51cf456fed9d14e4296b6e0bf19c8fd0f02bb
Merge "Use HTTP response codes for identity v3"