keystone haproxy service configuration is broken when using federation

As of ZED and any branch that backported support for federation on kolla, the generated service.cfg file for keystone isn't correct.

Indeed, for the backend balancing mode, the template do generate an improper value:

```
backend keystone_external_back
    mode http
    ['balance source']
```

where it should produce:

```
backend keystone_external_back
    mode http
    balance source
```
This is due to the way the service default main.yml substitution is made compared to other services:

On keystone:
```yaml
backend_http_extra: "{{ ['balance source'] if enable_keystone_federation | bool else [] }}"

```

vs

On horizon

```yaml
backend_http_extra:
  - "balance {{ 'roundrobin' if enable_memcached | bool or horizon_backend_database | bool else 'source' }}"
```

As keystone need source when federation is enabled and roundrobin when disabled, we should rather use:

```yaml
backend_http_extra:
  - balance "{{ 'source' if enable_keystone_federation | bool else 'roundrobin' }}"
```

Of course we could leave the balance empty when keystone federation is disabled, but as the default balance algorithm for any backend is roundrobin no matter what (out of higher rank default directive set), that would probably beneficial for everyone to get the value explicited rather than letting operators have a long search about it.

ref: https://cbonte.github.io/haproxy-dconv/1.8/configuration.html#4-balance