added clean tasks

@kpiwko 

solves #7 

added clean task

```
clean
    cleanInstallations
    cleanRepository
    cleanWorkspace
    init
    prepare-env
```

This is just default value of repository - so this is not right https://github.com/arquillian/arquillian-spacelift-gradle-plugin/blob/master/src/main/groovy/org/arquillian/spacelift/gradle/SpaceliftExtension.groovy#L22