Refactor backup/restore

[kiranchitturi]

• Example commands:

fab backup_to_s3:kcloud,system_history

 * backup system_history collection to S3
 * Stored in S3 under path s3://sstk-dev/solr-backups/kcloud_system_history
 * the index directory is tarred up per shard and uploads to S3 bucket
 * Replicas are ignored, only index dirs of leaders are considered

fab restore_backup:kcloud,system_history,test

* restore system_history index to collection test
* Only works if the backup data and restore collection have the same number of shards
* SolrCloud should be down while restoring the data (TODO: Restart each shard while restoring data)