Update ERT to support deployment on GCP as well

[anighanta]

Currently ERT is configured to only run on AWS with SNS and lambda features used as default in the code.

We need to add the flexibility to

• Create 2 entry points, one for each of AWS and GCP. When ERT triggered through AWS entry point we should be able to read the config from config file path mentioned in the environment variables

• Add support in ERT so that it can communicate with GCP pub/sub as well along with AWS's SNS notifications

• Connect to the relational database on GCP to perform ERT database operations [blocked by Create a SQL database and appropriate Schema in GCP #184]