Variable expansion in maven goals

Hi

I had a Jenkins job, where I needed to calculate goals to be executed in a shell script. So I two pre-build steps:
- shell script
- inject environment variables (EnvInject)

Then I wanted to put that variable into maven goals, like this:

```
-o clean install -pl ${some_variable}
```

This did not work. This change fixes that. 

Would be happy to know if this is ok, or should be fixed in some other way.

Regards
Marcin