Don't install solum dashboard if solum disabled

Even if i have `solum: False` in `vars.yml` vagrant provisioning install solum dashboard. As a consequence horizon is not working (500 error when trying to access to the login page).

```
[Thu Nov 13 23:39:35.442157 2014] [:error] [pid 17856:tid 139905715730176] [remote 82.230.242.143:35932]     import solumdashboard.assemblies.views as views
[Thu Nov 13 23:39:35.442168 2014] [:error] [pid 17856:tid 139905715730176] [remote 82.230.242.143:35932]   File "/opt/stack/solum-dashboard/solumdashboard/assemblies/views.py", line 19, in <module>
[Thu Nov 13 23:39:35.442185 2014] [:error] [pid 17856:tid 139905715730176] [remote 82.230.242.143:35932]     from solumdashboard.api.client import client as solumclient
[Thu Nov 13 23:39:35.442196 2014] [:error] [pid 17856:tid 139905715730176] [remote 82.230.242.143:35932]   File "/opt/stack/solum-dashboard/solumdashboard/api/client.py", line 18, in <module>
[Thu Nov 13 23:39:35.442214 2014] [:error] [pid 17856:tid 139905715730176] [remote 82.230.242.143:35932]     from solumclient import client as api_client
[Thu Nov 13 23:39:35.442240 2014] [:error] [pid 17856:tid 139905715730176] [remote 82.230.242.143:35932] ImportError: No module named solumclient
```

My fix is dirty but this is the more straightforward. In my opinion,