On KV nodes continuous backup agent should be run

On all nodes with the data service the continuous backup agent - cbcontbk - should be started by the babysitter. It should be provided:

• "backup" command
• CBAUTH_REVRPC_URL- so we can use cbauth to authenticate with the server
• --admin-port - the admin port of the node (e.g. 8091)
•  --username - the username to be authenticated with. Suggestion: @ns_server or @continuous_backup (the former is what cbimport expects).
• --stats-port - the http port cbcontbk should expose Prometheus metrics on (9125)
• --data-dir - the path to the backup path in the data directory