build status

As suggested by @daspilker for https://github.com/jenkinsci/job-dsl-plugin/pull/251, I split this into a separate pull request.

## Build Status Condition

The build status condition was previously built using a SimpleCondition. Unfortunately, this generated the wrong XML for the Run Condition Plugin version 1.0: it was generating blobs of the form:

```
<condition class="org.jenkins_ci.plugins.run_condition.core.StatusCondition">
  <worstResult>SUCCESS</worstResult>
  <bestResult>FAILURE</bestResult>
</condition>
```

Whereas using the gui to configure a job generated XML of the form:

```
<condition class="org.jenkins_ci.plugins.run_condition.core.StatusCondition" plugin="run-condition@1.0">
  <worstResult>
    <name>FAILURE</name>
    <ordinal>2</ordinal>
    <color>RED</color>
    <completeBuild>true</completeBuild>
  </worstResult>
  <bestResult>
     <name>SUCCESS</name>
     <ordinal>0</ordinal>
     <color>BLUE</color>
     <completeBuild>true</completeBuild>
  </bestResult>
</condition>
```

Through further experimentation, we found that the `ordinal` tag was necessary and sufficient for jenkins to correctly process the job. Unfortunately, this meant we couldn't use a `SimpleCondition` for build status anymore, as the testing infrastructure wasn't able to handle `SimpleCondition` nodes with children.