Set environment in the Salt ENC output for Salt's salt.tops.ext_nodes

The output should be like

```
---
classes:
- class1
- class2
- class3
environment: prod
parameters:
  puppetmaster: ''
  domainname: ''
  hostgroup: <TEXT>
  root_pw: <TEXT>
  foreman_env: prod
  salt_master: <TEXT>
```

The Salt master top module salt.tops.ext_nodes can use it in https://github.com/saltstack/salt/blob/develop/salt/tops/ext_nodes.py#L83 to tell Salt in which environment to look for the states.

```
# salt 'minion1.*' state.show_top                                                                                                                                      
Executing job with jid 20140920211440447223
-------------------------------------------

minion1.domain.de:
    ----------
    prod:
        - class1
        - class2
        - class3
```

Salt's default environment is always "base".