Add support for instance labels on GCP

In order to better track running costs, we want to be able to add labels to the GCP instances created by docker-machine, which we use internally.

This merge request adds a new command line flag to the GCP driver, --google-label, with a format key:value. This label can be set multiple times to set multiple labels

See #11 (closed)

QA

• Download the built binary from the artifacts
• Authenticate locally with your "application default" credentials gcloud auth application-default login
• Run ./docker-machine-Darwin-x86_64 create -d "google" --google-labels "key1:value1,key2:value2" --google-project group-verify-df9383 test
• Verify that there are indeed labels on the instance

• Remove the instance ./docker-machine-Darwin-x86_64 rm test