Add variable to choose experiment allowing customized environment

Add experiment_in variable with 'atlas' as defualt. Variable determines
which gird-setup script will be used to set the environment. Needed for
belle specific evars which are different from ATLAS.

Changes:
    modified:   gce_node/manifests/grid_setup.pp
    modified:   gce_node/manifests/init.pp
    renamed:gce_node/templates/grid-setup.sh.erb ->
    gce_node/templates/grid-setup.sh.atlas.erb
    new file:   gce_node/templates/grid-setup.sh.belle.erb