Great_expectations: Multiple environment variables under a single key fail to substitute

[Aylr]

Describe the bug
Multiple environment variables under a single key fail to substitute

To Reproduce

1. create a datasource with a connection string that pulls in multiple environment variables.

latest/features/datasource.html
datasources:
  staging:
    credentials:
      connection_string: "bigquery://${PROJECT}/${DATASET}"
    class_name: SqlAlchemyDatasource
    module_name: great_expectations.datasource
    data_asset_type:
      module_name: great_expectations.dataset
      class_name: SqlAlchemyDataset
 ```
2. Set an environment variable `export PROJECT=my_secret_project_1234` and another `export DATASET=foo`.
3. Run `great_expectations datasource list`
4. Note the incomplete substitution

1 Datasource found:

• name: staging
  module_name: great_expectations.datasource
  class_name: SqlAlchemyDatasource
  credentials:
  connection_string: bigquery://my_secret_project_1234/${DATASET}
  data_asset_type:
  module_name: great_expectations.dataset
  class_name: SqlAlchemyDataset
  ```
  
  
  
  1. Run any feature that uses the datasource and note the backend specific error about failing to connect to a database.
  
  

Expected behavior
I expect all environment variable substitutions to be substituted.

Environment (please complete the following information):

• first seen on ubuntu w/ GE 0.11.1
• reproduced on macOS w/ GE 0.11.4

[Aylr]

@anthonyburdi and I discovered and reproduced this together.

[anthonyburdi]

I'm currently working on a fix to this & a related bug and should have a PR soon. (branch)

[github-actions[bot]]

Is this still relevant? If so, what is blocking it? Is there anything you can do to help move it forward?\n\nThis issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.