Allow custom variables for deployment environment

Issue Summary

Deployment variables only accept values explicitly pre-defined in the deployment section according to our documentation. Users would like to be able to setup environments automatically using environment variables or just select an already created environment without editing the YAML file.

Steps to Reproduce

1. Setup an environment variable called TARGET_ENV
2. Edit the YAML file with below example code

   custom:
    variables:
    name: $TARGET_ENV
    step:
    deployment: $TARGET_ENV
    caches:
    node
    script:
    echo "Deploying to " $TARGET_ENV
    /bin/bash scripts/deploy.sh $TARGET_ENV
   

Expected Results

Create a deployment environment and its variables if it still does not exist by defining it on the yaml file, either through an environment variable or giving it providing the deployment tag the name of a deployment environment that currently does not exist.

Workaround

Use the hard-coded deployment environment name as the environment name in the YAML file.