This patch introduces support for running custom validations by changing
the behavior of the validations actions ListValidationsAction,
ListGroupsAction and RunValidationAction.
Until now, these actions sourced validations from a directory on disk.
Now, these action are sourcing validations from the plan container
subdirectory (custom validations), or, if this is not available, from
the Swift container holding the default validations.
Change-Id: I9e9131b355312c53f12d154976d5d9cd706cc338
Implements: blueprint custom-validations
Depends-On: I338e139fa770ebb7bdcc1c0afb79eec062fada8b