Read InfluxDB credentials from a secret

[jkhalack]

Description

Right now the only way to pass influxdb credentials to pravega (needed to store pravega metrics) is via pravega_options block of the custom pravegacluster resource.
As a result the above credentials are hanging around in plain text and are available via kubectl get pravegacluster -o yaml command.
It would be much better if pravega-operator had an option to take a name of the influxdb secret that holds the credentials (assuming that influxdb is deployed in the same namespace as pravega) and read the influxdb credentials from there.

Importance

Having the influxdb credentials in plain text in the CR poses significant security risk.

Location

https://github.com/pravega/pravega-operator/blob/master/pkg/apis/pravega/v1alpha1/pravega.go

Suggestions for an improvement

Add a new influxdb_secret spec to pravega spec, that would take the name of influxdb secret, read database credentials from the secret, then create

• controller.metrics.influxDBUserName and controller.metrics.influxDBPassword options for pravega controller
• metrics.influxDBUserName and metrics.influxDBPassword options for pravega segmentstore