Rename event names to match what's in command table

This is necessary for any plugins that modify the command/arg tables
to catch the events that are fired.
Split RDS modify command into add/remove commands

The modify-option-group has been split into:

* add-option-to-option-group
* remove-option-from-option-group

Part of this change included the ability for the argument name
to be changed by an external client.  This is encapsulated in
the awscli.customizations.utils.rename_argument function which
I've added.
Use the param obj name for the help docs

That way we pick up any renames that happened through
the arg table.
Update tests with new help command interface
Remove unused classes
Incorporate review feedback

Also remove the modify-option-group from the command table.
Merge branch 'rds-split' into develop

* rds-split:
  Incorporate review feedback
  Remove unused classes
  Update tests with new help command interface
  Use the param obj name for the help docs
  Split RDS modify command into add/remove commands
  Rename event names to match what's in command table