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

Change-Id: I5f9a3ce76c233e62ff6addcaf6d229455ac29374

requirements.txt

@@ -5,3 +5,5 @@ oslo.log>=3.36.0 # Apache-2.0
 pbr!=2.1.0,>=2.0.0 # Apache-2.0
 Babel!=2.4.0,>=2.3.4 # BSD
 python-designateclient>=2.7.0 # Apache-2.0
+
+horizon>=13.0.0 # Apache-2.0

tox.ini

@@ -17,7 +17,6 @@ setenv = VIRTUAL_ENV={envdir}
 whitelist_externals = find
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-       http://tarballs.openstack.org/horizon/horizon-master.tar.gz
 
 commands =
     find . -type f -name "*.pyc" -delete