wic: flatten imager class hierarchy

wic: flatten imager class hierarchy

wic code is hard to follow due to deep and twiggy class
inheritance tree.

Flatten imager tree:
 wic -> wic_create -> Creator -> DirectPlugin -> DirectImageCreator
to
 wic -> wic_create -> DirectPlugin
by
 removing Creator class and creator module
 merging DirectImageCreator into DirectPlugin

Changed APIs to use the same parameters names.

Passed parsed command line options as an object down the stack.

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>