add pull-deps command for plugins

[knoguchi]

For now, please include necessary extensions under extensions directory when you make the parcel. As of now the extensions are loaded from the /opt/parcels/DRUID directory. To make changes to the extension load list does not automatically pull the extensions and their dependencies.

I don't think it's feasible to create a giant parcel that includes all the extensions. More over I have a custom extension that is not part of Druid distribution. I need a way to "add on" extensions. pull-deps can pull extensions from Maven repositories.

http://druid.io/docs/latest/operations/pull-deps.html

Where to save the extensions?

• /opt/parcels/DRUID/extensions is good so all the roles can share the extension. But I'm not sure cloudera-scm user can write to the directory.
• to create a separate parcel might be technically right way, but it's a maintenance nightmare.
• per process directory would work but to pull extensions at config change is real pain e.g. /var/run/cloudera-scm-agent/processes/XXX-druid-overlord/..