Install horizon directly from pypi

It's no longer possible to install horizon with our current
method, and it's causing the ci to fail.
http://lists.openstack.org/pipermail/openstack-dev/2018-March/128310.html

Depends-On: https://review.openstack.org/553703
Change-Id: If2923cb97514fb79e25a6b40b287548bf6516416

requirements.txt

@@ -10,5 +10,7 @@ httplib2>=0.9.1 # MIT
 python-keystoneclient>=3.8.0 # Apache-2.0
 PyYAML>=3.12 # MIT
 
+horizon>=13.0.0 # Apache-2.0
+
 # Watcher-specific requirements
 python-watcherclient>=1.1.0 # Apache-2.0

tox.ini

@@ -21,7 +21,6 @@ whitelist_externals = /bin/bash
                       find
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-       http://tarballs.openstack.org/horizon/horizon-master.tar.gz
 commands =
     rm -f .testrepository/times.dbm
     find . -type f -name "*.pyc" -delete