Fix output of RS_DESTROY when set to 'no'

When RS_DESTROY is "no" or "false" the value for RSpec.configuration.rs_destroy is nil and the debug output then looks like this:

```
Setname:             centos-64-x64
Configuration:       {"nodes"=>{"main.vm"=>{"prefab"=>"centos-64-x64"}}}
Virtual Environment: vagrant_virtualbox
Default node:        main.vm.osg
Destroy node:
```

After fix:

```
Setname:             centos-64-x64
Configuration:       {"nodes"=>{"main.vm"=>{"prefab"=>"centos-64-x64"}}}
Virtual Environment: vagrant_virtualbox
Default node:        main.vm.osg
Destroy node:        false
```

This may be a bug in how RSpec handles assigning a new setting's default value to false.  If assigning a default value of false worked as expected then the code could be simply `ns.destroy.to_s`.