Add default task_id management by using @default_from_global

* Scenario methods now use @with_default_task_id which initialize
the task_id parameter: it sets task_id to the 'RALLY_TASK' env var
available in the global file ~/.rally/globals
* Added @with_default_task_id global to use @default_from_global
decorator with task_id, so we can use directly @with_default_task_id
and don't have to use @default_from_global that requires parameters.

Implements blueprint: cli-use-task-subcommand

Change-Id: I3fdd80b0fa89354ba24aa78b2b7db84d8a6d58c1