[stable/datadog] DD_DOGSTATSD_PORT typo

[johnkost]

There is a typo the datadog-agent environment variables that doesn't allow overriding the statsd port.
DD_DOGSTATS_D is spelled DD_DOGSTATD_PORT.

https://github.com/DataDog/datadog-agent/blob/master/pkg/process/config/config.go#L401
https://github.com/helm/charts/blob/master/stable/datadog/templates/container-agent.yaml#L29

Version of Helm and Kubernetes:

helm version
Helm 3.0.2
OpenShift 3.11 (Kubernetes 1.11)

Which chart:
stable/datadog

What happened:
The daemon set fails to override the statsd port so it always uses the default

What you expected to happen:
The statsd port to be overridden so I can send metrics to it.

How to reproduce it (as minimally and precisely as possible):
Override .datadog.dogstatsd.port and send metrics to the new port

Anything else we need to know: