escape XML when substituting variables into Beaker job XML

In our Beaker CI we have a Jenkins job which submits a Beaker job like this:

```
...
    <whiteboard>
        beaker dogfood for Gerrit change ${GERRIT_CHANGE_NUMBER} patch ${GERRIT_PATCHSET_NUMBER}: ${GERRIT_CHANGE_SUBJECT}
    </whiteboard>
...
```

If we are testing a commit which has XML in the commit message, this will be interpolated directly into the job XML and can cause an XML syntax error (or other undesired behaviour).