Feature: Disable tasks

Currently the only way to disable tasks is to remove from S3 the file that define the task. Be able to disable tasks in the task definition could be useful.

Proposals:

1. Add new attribute in task definition to indicate if the task is enabled or disabled:
    * enabaled: \[True|False\] (True by default)
    
1. Replace cron expression with a keyword, ie. **disabled**