A couple fixes and tweaks

 - Prefix cloud machine names with 'vagrant_' to distinguish them from other cloud servers on account
 - Change flavor match to /1 GB/, to change from the '1GB Standard Instance' match to the new '1 GB General Purpose v1' flavor
 - Rename Regex replacement for server.image from --RS-FLAVOR-- to more appropriate --RS-IMAGE--
 - Allow script to run from directories other than the script's directory, by finding the absolute path for Vagrantfile.template