Add support for legacy runlevel autoyast profiles (bnc#865347)

* added new desktop file as the entry `X-SuSE-YaST-AutoInstClient` is significant for selecting the correct autoyast client proposal
* enhanced schema
* tests

## New proposed profile for services

```xml
<services-manager>
    <default_target>multi-user</default_target>
    <services>
      <enable config:type="list">
        <service>at</service>
        <service>cron</service>
        <service>nscd</service>
        <service>openct</service>
        <service>postfix</service>
        <service>rsyslog</service>
        <service>sshd</service>
      </enable>
      <disable config:type="list">
        <service>libvirtd</service>
      </disable>
    </services>
  </services-manager>
```
## Legacy profiles are these:
### Runlevel profile
```xml
<runlevel>
  <default>3</default>
  <services config:type="list">
    <service>
      <service_name>sshd</service_name>
      <service_status>enable</service_status>
      <service_start>3</service_start>
    </service>
  </services>
</runlevel>
```

### Plain list of services

* the issue here was missing services that a user wants to disable

```xml
  <services-manager>
    <default_target>multi-user</default_target>
    <services config:type="list">
      <service>at</service>
      <service>cron</service>
      <service>nscd</service>
      <service>openct</service>
      <service>postfix</service>
      <service>rsyslog</service>
      <service>sshd</service>
    </services>
  </services-manager>
```