Add environment-handling to roles.

A new subdirectory, 'environment', is available for roles. The dictionary elements from the main.yml file within are merged with those from previous roles. (Later roles override previous ones.) Finally, the play's environment is merged into the roles' environment variables.

Thus, the (lowest-to-highest) precedence for any environment variable specified in multiple places is: roles (in role-load order) - play - task.