Correct ros installation and use of ros variables.

If ROS is not installed, it will be installed by
./install ros 2
The absence of ROS will not stop the script for
the 4 first steps of the installation.

Also remove useless ros variables. The redefinition
of ROS_ROOT was wrong for groovy and fuerte anyways.
Merge pull request #7 from francois-keith/master

Correct ros installation and use of ros variables (close #6).