Modifies java element to support custom install location

This patch modifies the common "java" element in order to allow
  users to specify the directory that will contain the JDK install.
Currently, the java element assumes an install path of "/usr/java".
  Some distributions, such as HDP, use a different directory location
  for the JDK install.
This patch introduces a new environment variable, JAVA_TARGET_LOCATION,
  that can be set in order to override the location where the JDK
  will be installed.  This current patch adds this support for .bin
  install types only.
This patch also updates the README file to provide more information
  about the JAVA_TARGET_LOCATION environment variable.
Once this patch is approved, I plan on submitting subsequent patches to
  use the "java" element in the HDP image generation scripts, rather than
  the custom scripts used by hadoop-hdp currently.

Change-Id: Ia0d26baed5e32d05699e71b9ee7ce489cd5a11c9