Add ability to constrain an option to a set of values

[outofcoffee]

Example

actions:
  deploy-services:
    driver: jenkins
    jobId: "example"
    template: deploy services {version} to {environment}
    options:
      foo:
        constraints:
          list:
            - bar
            - baz

Other possibilities:

actions:
  deploy-services:
    driver: jenkins
    jobId: "example"
    template: deploy services {version} to {environment}
    options:
      foo:
        constraints:
          pattern: "[a-zA-Z0-9]+"

Notes

Error message should list allowed values.