doc: Update admin_guide for hadoop.

Current version of admin_Hadoop.md contains an 
obsolete set of instructions.Changes are purely 
textual; no code changes.Linking to a community 
site extends the viability of the doc and allows 
instructions to keep pace with future versions.
Instructions have been validated.

Change-Id: Ie330b2a23513ae140d3cd5f19af3045fff3bc9d5
BUG: 1132105
Signed-off-by: jonKSA <cope.jonathan@gmail.com>
Reviewed-on: http://review.gluster.org/8505
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>