webadmin: Gluster hooks - view content popup

A new action 'View Content' added to Gluster Hooks sub tab
in cluster. This will open a popup which will show the content
of the hook.

If the hook is a binary file, 'View Content' action will be disabled.

Change-Id: Ic304b8155d5bf769813197a9796bf6d86fd10bfe
Signed-off-by: Kanagaraj M <kmayilsa@redhat.com>