Allow specifying environment variables in the Influx CRD spec.

[bheesham]

There are parts of the ConfigMap that should be Secrets, such as the:

• INFLUXDB_HTTP_SHARED_SECRET;
• INFLUXDB_HTTPS_PRIVATE_KEY;
• INFLUXDB_HTTPS_CERTIFICATE.

See also: https://github.com/influxdata/influxdata-operator/issues/10#issuecomment-515653867