Add os-testr in test-requirements.txt
Add os-testr in test-requirements.txt

The py27 and py35 jobs fail because the subunit-trace command is not
found. The command is utilized in tools/pretty_tox.sh. But os-testr
is not included in test-requirements.txt. So add it to fix the gate
job failures.

Change-Id: Ifdd4bad354aebf24e37769ff8e1e5f3bdbeebd76
Related-Bug: #1753898

openstack/python-designateclient