Fix failure highlighted on OpenSUSE Tumbleweed, where the `cmd.run` runs
before the `pkg.installed`:

* https://gitlab.com/saltstack-formulas/nginx-formula/-/jobs/1345325819#L2830

* Close #241

```bash
Examining nginx/certificates.sls of type state
[206] Jinja variables should have spaces before and after: {{ var_name }}
nginx/certificates.sls:40
    - contents_pillar: {{salt['pillar.get']('nginx:certificates:{}:public_cert_pillar'.format(domain))}}

[202] Jinja statement should have spaces before and after: {% statement %}
nginx/certificates.sls:47
{% if salt['pillar.get']("nginx:certificates:{}:private_key".format(domain)) or salt['pillar.get']("nginx:certificates:{}:private_key_pillar".format(domain))%}

[206] Jinja variables should have spaces before and after: {{ var_name }}
nginx/certificates.sls:54
    - contents_pillar: {{salt['pillar.get']('nginx:certificates:{}:private_key_pillar'.format(domain))}}
```

BREAKING CHANGE: all previous `nginx` based configurations must be reviewed;
`nginx.ng` usage must be promoted to `nginx` and any uses of the original
`nginx` will have to be converted.

... by providing a pillar string. I developed this for use in
combination with ext_pillar and file_tree to deploy letsencrypt
certificates.

default value if dhparam isn't defined in the pillar.

In order to improve security and ease of use, added creation/generation
of dhparam file.