Pass the component list as the pre-hook stdin

As discussed during the workshop, it allows to define policies for
component executions.

The list of components is passed as a JSON object.  I have tried to make
it easy to extend.
Record the state of the blocked components
Document the format of stdin in the pre-hook
Merge pull request #10 from piojo/hooks-input

Pass to the pre-hook the components to run
Merge pull request #1 from quattor/master

Update