Respect credential_file and token args on GCE
Respect credential_file and token args on GCE

Skip permissions check for GCE VM service account when a credential_file
or token were passed as arguments. Fixes #26

ankushagarwal/cloudsql-proxy,digiexchris/cloudsql-proxy,pdecat/cloudsql-proxy,digiexchris/cloudsql-proxy,ankushagarwal/cloudsql-proxy,pdecat/cloudsql-proxy,GoogleCloudPlatform/cloud-sql-proxy,GoogleCloudPlatform/cloud-sql-proxy