Fixes #39 - Add ros_packages to ros_install role. 

Not sure why I had to do the loop + temp variable with echo...

If using 
```
- name: Install specific ROS packages
  when: full_ros_packages_names is defined and full_ros_packages_names != ""
  apt: name="ros-{{ros_release}}-{{item}}"
  with_items: ros_packages
```

vagrant tries to run: `apt-get install ros-hydro-first-package second-package` (prefix applied to first package only!!


The actual list of additional packages will be added to #41