Enable running disk-image-create on SUSE Linux

- Ensures /sbin and friends are in $PATH when invoked (without this,
  various sudo invocations fail in exciting ways).
- Use dib-run-parts in lib/common-functions instead of run-parts
  (neither SLES nor openSUSE ship run-parts).
- Ensure dib-run-parts doesn't descend into subdirectories (same
  behaviour as run-parts).
- Move dib-run-parts from root.d to bin (cleaner, consistent with
  other elements with separate bin scripts).
- Tested by building Ubuntu image on openSUSE 12.3.
- Note: this doesn't add support for creating SUSE images, it just
  lets you run disk-image-create on SUSE-based distros.

Change-Id: I906c6bc3cf51cdf2c4415adeae1ca250faac25e1
Merge "Enable running disk-image-create on SUSE Linux"