Actions/commands improvements and fixes

[aplowman]

• refactor tests of Command.get_command_line to use a utility function
• fix hashing of commands (environment specifiers are now distinguished)
• document variables that can appear in commands and environments
• enable an alternative, more compact form of action-environment specification within task schemas:

  actions:
    - environments:
        processing: python_env
        main: sim_env
      commands:
        - command: echo "hello"

  instead of:

  actions:
    - environments:
        - scope:
            type: processing
          environment: python_env
        - scope:
            type: main
          environment: sim_env
      commands:
        - command: echo "hello"