fixes for gcutil-1.10.0 and v1beta16
tested PE3.1 on debian-7 with v1beta15 and v1beta16
use existing PD boot disk on instance create
Merge branch 'v1beta16' of https://github.com/erjohnso/puppetlabs-gce_compute into v1beta16
improved PD documentation
fixes #17, fixes #18; updated for gcutil-1.10.0/v1beta16
Merge pull request #19 from erjohnso/master

Updated to support GCE API v1beta16 (gcutil-1.10.0)