Install horizon from git

Install horizon in OpenStack CI from git instead of using tarball, this
allows depends-on to work.

Update tox.ini and requirements.txt for proper setup.

Change-Id: I473a7d378926f9583d695d78da661f58f03ce30e

requirements.txt

@@ -3,3 +3,7 @@
 # process, which may cause wedges in the gate later.
 Django<2.0,>=1.8 # BSD
 python-freezerclient>=1.3.0 # Apache-2.0
+
+# OpenStack CI will install the following projects from git
+# if they are in the required-projects list for a job:
+horizon>=13.0.0  # Apache-2.0

tox.ini

@@ -11,10 +11,9 @@ setenv = VIRTUAL_ENV={envdir}
          NOSE_OPENSTACK_RED=0.05
          NOSE_OPENSTACK_YELLOW=0.025
          NOSE_OPENSTACK_SHOW_ELAPSED=1
-install_command = pip install -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt} -U {opts} {packages}
-deps = -r{toxinidir}/requirements.txt
+deps = -c{env:UPPER_CONSTRAINTS_FILE:https://git.openstack.org/cgit/openstack/requirements/plain/upper-constraints.txt}
+       -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
-       http://tarballs.openstack.org/horizon/horizon-master.tar.gz
 commands = python manage.py test {posargs}
 
 [testenv:pep8]