Add support for creating a graph that points at an external hbase table/es index

This is done by posting doing:

curl \
  -XPOST \
  -H 'content-type: application/json' \
  /api/projects/<project-id>/graphs \
  -d '{
    "properties":{
      "storage.backend":"local",
      "storage.directory": "/tmp/foo"
    }
  }'