Updates SELinux configuration in HDP images

Recent changes to the core diskimage-builder elements
  have caused some previous warnings/errors that were silent
  to surface.  These errors are breaking the generation of the
  CentOS HDP images.
The failure is caused by the fact that the HDP images did
  not have all the correct configuration in place to disable
  SELinux.  Disabling SELinux is required for the HDP platform.
This patch modifies the dependencies for the hadoop-hdp
  element in order to include a dependency on the
  disable-selinux element from diskimage-builder.
This patch removes the original SELinux configuration commands
  for HDP, since these were found to be incorrect.
This patch adds the disable-selinux element to the HDP
  plain image as well.
This patch also includes some element refactorings, moving
  some elements into element-deps for hadoop-hdp, since these
  are required elements for HDP.

Change-Id: Ie6e26b397d022ea813c5022071bf3752ab39fa82
Merge "Updates SELinux configuration in HDP images"