Fix Status-Line in HTTP response

Fix Status-Line in HTTP response

There is a strict rule about constructing status line for HTTP:
'...Status-Line, consisting of the protocol version followed by a
numeric status code and its associated textual phrase, with each
element separated by SP characters'
(http://www.faqs.org/rfcs/rfc2616.html)

This patch coerces filling associated textual phrase.

Also removed unused code from cinder/tests/unit/test_exception.py

Change-Id: Ia9099fb5020cee02bfee2cd0e8e111845918c36c
Closes-Bug: #1496055
Co-Authored-By: Marian Horban <mhorban@mirantis.com>