doc: Hadoop clarifications

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
doc: start Hadoop installation docs

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
doc: add place holder for libcephfs-java install docs

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
doc: document new hadoop config options

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>
doc: update Hadoop docs with plugin download

Signed-off-by: Noah Watkins <noahwatkins@gmail.com>