Add support for mounted secrets

[anilarora]

With the telegraf 1.27 release, there is support for reading the credentials from a secrets mounted on the file system. influxdata/telegraf#13035. Would like to be able to utilize a kubernetes secret for credentials, which would allow for the configuration to be stored in a configmap rather than a secret. (Another issue will be entered to see if we can support configuration stored in a configmap.)

Relevant URLs

influxdata/telegraf#13035

What products and version are you using?

Telegraf operator 1.3.8