update webtest version in test-requirements

[Kui Shi]

run run_tests.sh or tox -epy27 may get many failures.
The reason is that the webtest is not the latest version. webtest check the environment:

In latest webtest:
assert type(environ[key]) in METADATA_TYPE, (
             "Environmental variable %s is not a string: %r (value: %r)"
             % (key, type(environ[key]), environ[key]))

In old webtest:

assert type(environ[key]) is str, (
             "Environmental variable %s is not a string: %r (value: %r)"
            % (key, type(environ[key]), environ[key]))

The latest webtest is 2.0.7, designate it in test-requirements.txt.

[Dolph Mathews (dolph)]

You'll have to unpin the global requirement first https://github.com/openstack/requirements/blob/bdc34443a397aa1d280741dbcccfd5e69b7addee/global-requirements.txt#L137

[Kui Shi (skuicloud)]

I have submitted two patches using the same bug id:

https://review.openstack.org/#/c/44697/
https://review.openstack.org/#/c/44773/