Fix the sudo rules for unpacking fedora raw images.

Change-Id: Icba75e0380f63a8f6059d1be4c63ac823b9f473d
Prevent silent failure of element-info:

bin/element-info accesses it's library via a symlink:
  bin/diskimage_builder -> diskimage_builder

This causes the relative path in elements.py to be
incorrect.

element-info silently fails in this situation, because
it allows missing 'element-deps' files, so that these files
may be optional.

This change causes element-info to fail explicitly if $ELEMENTS_DIR
is not set, as it now is when called by diskimage-create, and
adds tests reflecting this behavior.

Change-Id: Iec50f934feb13dfed64d69297a3af6ac9f842677
Merge "Prevent silent failure of element-info:"