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

(From OE-Core rev: a1e24c4a5f5771b7ad35e53ce96c6d82212e4d7e)

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

scripts/lib/mic/imager/baseimager.py

@@ -26,12 +26,10 @@ import re
 import tarfile
 import glob
 
-import rpm
-
 from mic import kickstart
 from mic import msger
 from mic.utils.errors import CreatorError, Abort
-from mic.utils import misc, grabber, runner, fs_related as fs
+from mic.utils import misc, runner, fs_related as fs
 
 class BaseImageCreator(object):
     """Installs a system to a chroot directory.