add check_terra_env task

[tomkinsc]

This adds a new task, check_terra_env, which returns a number of fields related to the execution environment for a submitted job, including the following:

    Boolean is_running_on_terra
    Boolean is_backed_by_gcp

    String google_project_id

    String workspace_id
    String workspace_name
    String workspace_namespace
    String workspace_bucket_path

    String input_table_name
    String input_row_id

    String top_level_submission_id

    File env_info # output of running 'env'
    File gcloud_config_info # output of 'gcloud info'

These fields can be used downstream for programmatic interaction with (Terra) backend systems.

A workflow is included to run the task in isolation, called dump_gcloud_env_info.