Failover to secondary resource for rsync / download / list APIs

[soichih]

When a child task tries to rsync from parent task and if parent task's resource is currently off-line, we should look for other resources (in resource_ids) and use them instead.

I believe we need to update following.

1. task handler's rsync source resource
   Iterate through resources as rsync source instead of just trying resource_id

2. /resource/download & /resource/ls
   I think we should create a proxy APIs (/task/download /task/ls) that iterate through available resource and update our UI to use this instead of directly downloading from /resource. We should even deprecate /resource/download and /resource/ls?

[soichih]

On both cases, I think it's important to assume that resource maybe temporary down or data is purged - and keep testing for other resources.