Removes default usage of root-passwd element from HDP image generation

This patch removes the root-passwd element from the list of elements
  in the HDP image generation process.
It was determined that having root-passwd included causes some problems on
  RHEL, and is also a potential security issue.
This patch modifies the default cause, such that root-passwd is not
  included in HDP images.  In order to support local debugging of
  generated images, this patch adds a "-d" (debug) flag that can be
  enabled in order to add root-passwd to the HDP images. If not
  specified, the debugging flag defaults to false (not
  including root-passwd).
This patch updates the usage log to explain the usage of the
  -d parameter.

Change-Id: I1692916a3d7c08b8a3b180913e6fca3547a2e2bc
Merge "Removes default usage of root-passwd element from HDP image generation"