InfluxDB charm should support backups

[Tom Haddon]

The InfluxDB charm should support backups. Initially it's okay if the restore process is manual, but eventually we'll want to add that as a charm action as well.

[Herb McNew (herb)]

See https://docs.influxdata.com/influxdb/v1.3/administration/backup_and_restore/

Key points:

* The above page talks about backing up the metastore as a distinct entity, but that isn't necessary. When you back up a particular database it gets the metastore along with it.

* You must create per-database backups. There isn't a way to dump all the databases to a single set of files.

* The command to dump a backup is "influxd backup -database <dbname> <filesystem-location>"

* Influx backups to the same location are incremental backups. I suspect we want to support both incremental and full backups (but perhaps only ship full backups offsite?)

* To enumerate the databases from the command line use "influx -execute 'show databases'"

[Paul Gear (paulgear)]

Initial version committed in https://code.launchpad.net/~paulgear/influxdb-charm/+git/influxdb-charm/+merge/332218 and available for testing at https://jujucharms.com/u/paulgear/influxdb/ (cs:~paulgear/influxdb-4).