Add optional network policy object for deploy of step-certificates

[sshipway]

Description

This update to the helm template for step-certificates allows the optional creation of NetworkPolicy rules.

For users who have a Default-Deny policy rule on their kubernetes cluster (as we do) the normal deploy of a LoadBalancer Service will not be accessible from outside, and a Policy rule is required to permit the traffic.

Documentation is added to the Readme file and the values.yaml for the new available values.

Example

networkpolicy:
  enabled: true
  allow:
  -  "10.0.0.0/8"

In addition a couple of minor yaml formatting changes to make our automated yamllint a little happier.

[CLAassistant]

All committers have signed the CLA.