Snap: Configure backup retention period

Description:

Starting with commit https://github.com/RocketChat/Rocket.Chat/commit/15a01eeb8405df37019883236b672379e20fe3ee the destination for backups is no longer $SNAP_DATA/backup.tgz but ${BACKUP_DIR}/rocketchat_backup_${TIMESTAMP}.tar.gz aka `${SNAP_COMMON}/backup/rocketchat_backup_${TIMESTAMP}.tar.gz``

Steps to reproduce:

call snap run rocketchat-server.backupdb via daily cron.

Expected behavior:

Old bahavior was overwriting $SNAP_DATA/backup.tgz keeping the storage requirements low.

Actual behavior:

New (and better) behavior is a backup file per day. This should be documented in the docs so the fairly trivial cronjob for cleaning the directory could be set up.

Another way would be a (perhaps even configurable) retention time in https://github.com/RocketChat/Rocket.Chat/blob/develop/.snapcraft/resources/backupdb .

Server Setup Information:

• Version of Rocket.Chat Server: 0.70.4
• Operating System: Ubuntu 16.04
• Deployment Method: snap
• Number of Running Instances: 1
• DB Replicaset Oplog: active
• NodeJS Version: v8.11.3
• MongoDB Version:

Additional context

Start by reading .snapcraft/resources/backupdb and the issue's described backup paths and cron usage. Determine whether the intended result is documentation for cleaning ${SNAP_COMMON}/backup or a configurable retention period, then verify that backups are retained according to the selected behavior.