Implemented variable substitution for nested structures in cumulusci.yml

[aditya-balachander]

W-14176826

Implemented functionality to support variable replacement in nested structures in the cumulusci.yml file such as:

task:
    deploy:
        options:
            path: $project_config.project__package__path
            transforms:
                - transform: find_replace
                  options:
                      patterns:
                          - find: foo
                            replace: $project_config.project__package__namespace

Here both variables are substituted by the appropriate value. Previously only the path would've been substituted