Add WebTest as a test dependency
Add WebTest as a test dependency

Pecan has made WebTest an optional dependency for testing only [1].
Since it is used for blazar unit testing, we need to add it to the
test-requirements.

The lower version bound is required by requirements-check.
It is (arbitrarily) taken from the Queens' upper-constraints.

[1] https://github.com/pecan/pecan/pull/140

Change-Id: I67b730a21676a8191756effec7e15ab1895e1c95

openstack/blazar,openstack/blazar