Enhance funcitonality of cbbackup, cbrestore, and their wrappers to allow specifying multiple buckets and have the order honoured

Currently cbbackup, cbrestore, and their wrappers, known throughout as backup tools allow specifying a single bucket as a source, or all buckets.

It is proposed to allow multiple bucket names to be provided, and run the backup tools on each bucket in the order the bucket names are passed to the tool.

Example

cbbackup http://host:8091 /backups -u Administrator -p password -b bucket_name1,bucket_name2,bucket_name3

will run backup on bucket_name1, then bucket_name2, and finally bucket_name3.