Need WEBROOT in local_settings.py file

Horizon relies on the WEBROOT setting in order to handle
static resources.

Change-Id: Ia6b8d3453fec70d5865d52843e2ae3c48c2a0865
Closes-Bug: #1438922

spec/horizon_spec.rb

@@ -222,6 +222,10 @@ describe 'openstack-dashboard::horizon' do
           end
         end
 
+        it 'does have webroot set' do
+          expect(chef_run).to render_file(file.name).with_content(/^WEBROOT = \'\/\'$/)
+        end
+
         it 'does not have urls set' do
           [
             /^LOGIN_URL =$/,

templates/default/local_settings.py.erb

@@ -9,6 +9,8 @@ from openstack_dashboard import exceptions
 DEBUG = <%= node["openstack"]["dashboard"]["debug"] ? "True" : "False" %>
 TEMPLATE_DEBUG = DEBUG
 
+WEBROOT = '<%= node['openstack']['dashboard']['webroot'] %>'
+
 <% if node["openstack"]["dashboard"]["login_url"] %>
 LOGIN_URL = '<%= node['openstack']['dashboard']['login_url'] %>'
 <% end %>