[ha] Re-enable nginx proxy for Horizon

[ha] Re-enable nginx proxy for Horizon

Starting with MCP 2019.2, Horizon was moved under haproxy in
Active/Active mode by default via upstream changes:
- Adding haproxy class for horizon [1];
- Cleanup nginx horizon sites by default [2];

This change re-enables the old behavior where Horizon is served by
nginx instead of haproxy.

While at it, fix missing support in salt-formula-apache for wsgi
`locations`, so Horizon dashboard can access '/static' resources
(e.g. CSS/images).

JIRA: FUEL-408

[1] https://github.com/Mirantis/reclass-system-salt-model/commit/81c4c21a
[2] https://github.com/Mirantis/reclass-system-salt-model/commit/a3b38f46

Change-Id: I9b35d5d0ce4e0b53dae808c2620a31ca80290b55
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>