Removes discover from test-reqs
Removes discover from test-reqs

The discover package is part of Python 2.7, so we don't need
to explicitely require it. It's by the way annoying for
distribution package maintainers, as the file has to be
patched.

Keystone team has decided to drop python 2.6 support[1],
so, it's safe to remove it from test-requirements.txt.

[1] http://eavesdrop.openstack.org/meetings/keystone/2015/keystone.2015-11-24-17.59.html

Related-Bug: #1519449
Co-Authored-By: Thomas Goirand <5f50a84c1fa3bcff146405017f36aec1a10a9e38@goirand.fr>
Change-Id: I5bb258a4b305c0084be50bc22fe7a0e6a4f65aad

klmitch/python-keystoneclient,klmitch/python-keystoneclient