Create a tox environment for running the unit tests against the lower
bounds of the dependencies.

Create a lower-constraints.txt to be used to enforce the lower bounds
in those tests.

Add openstack-tox-lower-constraints job to the zuul configuration.

Add missing dev packages for libxml and libxslt to the bindep config
file.

See http://lists.openstack.org/pipermail/openstack-dev/2018-March/128352.html
for more details.

Change-Id: I4a51633916661bdfa926eb231b8e482628887e90
Depends-On: https://review.openstack.org/555034
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

Those are not installed anymore by default and are needed to fix builds
of python modules.

Change-Id: I38a1f0ff1ce5a776ee981c200d5b55d6753d260b

Add bindep.txt file with packages needed to use the repository.

Change-Id: I409d33c60a2be1886918b7e1ed8e4c6f3ca346c2