wic: Remove rpm and grabber dependencies from BaseImageCreator

wic: Remove rpm and grabber dependencies from BaseImageCreator

BaseImageCreator is a base class for DirectImageCreator and others,
and imports rpm and grabber (which imports rpm).

The various plugins e.g. DirectPlugin import the creators and
therefore these dependencies, which manifest at run-time as e.g.:

  Warning: Failed to load plugin imager/direct_plugin: No module named
    rpm

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>