Remove warning for allow_virtual change of default value

If you follow the current tutorial as the allow_virtual default will be changed in future releases you find the next warning:

```
[root@learning ~]# puppet apply --noop sshd.pp 
Notice: Compiled catalog for learning.puppetlabs.vm in environment production in 0.47 seconds
Warning: The package type's allow_virtual parameter will be changing its default value from false to true in a future release. If you do not want to allow virtual packages, please explicitly set allow_virtual to false.
   (at /opt/puppet/lib/ruby/site_ruby/1.9.1/puppet/type.rb:816:in `set_default')
Notice: Finished catalog run in 0.37 seconds
[root@learning ~]# 
```

As a new joiner to puppet the tutorial should be warning free as it can scary new people.

The fix sets the default for allow_virtual to false and the warning disappears:
```
[root@learning ~]# puppet apply --noop sshd.pp 
Notice: Compiled catalog for learning.puppetlabs.vm in environment production in 0.49 seconds
Notice: Finished catalog run in 0.33 seconds
[root@learning ~]# 
```