Charm should create 'admin' account

[Haw Loeung]

Hi,

In order to enforce auth, InfluxDB requires at least one 'admin' account. I think the charm should create this, and then allow retrieval of credentials via juju actions.

We also need to write out ~nagios/.influx-auth with the 'admin' credentials as the nagios check needs this to run 'show diagnostics'.

[Jacek Nykis (jacekn)]

Bumping importance because it's related to monitoring (we need a way to automatically create account for nagios)

[Laurent Sesquès (sajoupa)]

This is also needed for backups.
In templates/influxdb-backup, the following line:
DATABASES=$(influx --execute 'show databases' --format=json | jq --raw-output '.results[0].series[0].values[][0]' | grep -ve '^_internal$')
should be updated to add --username [...] --password [...]