Improvements to ec2 autoscaling modules

* Added desired_capacity and vpc_zone_identifier to ec2_asg
* Use ec2_argument_spec() method and then remove unnecessary
  declarations from argument_spec
* Remove AWS_REGIONS declaration
* Rename block_device_mappings to volumes to be consistent with ec2
* Remove all pep8 warnings except line length and continuation indent
* Use updated module_utils/ec2.py to add profile and security_token
  support
* Remove mandatory arguments for delete to make launchconfig deletion
  work
* Handle existing launch configurations better
* Improve output information
* Improve documentation