Add path to installing puppet 3 on nodepool nodes

Instead of looking for an argument, change the install_puppet.sh script
to look for an environment variable to switch to installing puppet3.
Also export this environment variable in prepare_node.sh (albeit with a
default value that does not install puppet3).

Change-Id: I04bee2fa5bb70a4d763edb2f3d30f117cad5d65b
Merge "Add path to installing puppet 3 on nodepool nodes"