Make install_puppet.sh more robust against failure

I hit an issue with a system with curl *and* wget installed.  The curl
download failed, leaving a corrupt get-pip.py and thus the wget
download started (which then saved to get-pip.py.1).  The script then
tried to run the corrupt version.  Although re-running is uncommon
unless you're manually deploying the scripts, best to also check for
the file first or these commands both re-download to get-pip.py.X

Change-Id: I6b70b4b7bb3d963ba70d71388c701951932e9adb
Merge "Make install_puppet.sh more robust against failure"