gcp client for faaskeeper

[EricPyZhou]

This PR should be merged after #13 as it contains the commits of it.

• add necessary libraries in the requirements.txt
• Add GCP config with gcp-specific configs, corresponding values can be found in the "gcp" in gcp_config_gcp.json in the faaskeeper repo.
• provide gcp implementation of the operations required in the queue system.