Add -U to pip install command in tox.ini

Ask pip to upgrade packages in the virtualenv so that a long-lived
virtualenv is still kept up to date as external dependencies change.

Change-Id: I0256cf4cc3aa7675956762dcf056e2709d36813a
Pin Sphinx to <1.2 in test-requirements

Sphinx was pinned to <1.2 in neutron requirements and we also
need to pin sphinx in neutronclient test-requirements.

Change-Id: I923cd19b3c28f9784a746243a5b3f76f7c38524c
Merge "Add -U to pip install command in tox.ini"