Running custom-metrics-stackdriver-adapter as non-root user

Adding a securityContext with non-root settings causes an error:

```yaml
securityContext:
allowPrivilegeEscalation: false
runAsNonRoot: true
runAsUser: 1000
```

allowPrivilegeEscalation: false is fine but runAsNonRoot, runAsUser appear to cause the error:

```
Unable to get StackdriverAdapter apiserver config error creating self-signed certificates: mkdir apiserver.local.config: permission denied
```