Support 15.7 variables spec

Variables at the top and job level can be expanded when the pipeline is created, eg:

variables:
  SHAPE: red
  TITLE: the $SHAPE house

In 13.7 and later variables can take the form of a map. 13.7 defined pre-fill variables for manual pipeline interfaces:

variables:
  SHAPE: red
  VARNAME:
    description: My Web UI description
    value: the value of it

15.7 introduced the map flag "expand" to control if a variable is expanded by gitlab or set verbatim

variables:
  SHAPE: red
  VARNAME:
    expand: false
    value: print $SHAPE

By default, or with expand=true, $SHAPE would be pre-expanded to red in top-level and job variables.

15.7 introduced "options" to give pre-fill choices for the web pipeline

variables:
  SHAPE:
    description: Choose a shape
    value: circle
    options:
      - square
      - circle
      - triangle