add support for environment variables

[vanHavel]

According to the CoreDNS docs, environment variables can be used in the Corefile. This might make a few things easier to configure, as the server config can be defined in values.yaml and small overrides can be done using environment variables in other values files.

This PR adds supports for environment variables to the CoreDNS deployment.

[vanHavel]

After creating the PR, I saw that there is already #90 which implements the same feature. The only difference is that #90 uses lists, while this uses maps to represent the env vars.

Personally I find maps to be slightly preferable, as they allow easier overrides, e.g. with --set.

[hagaibarel]

Hi, thanks for addition, definitely something we should support.

I think your approach will limit the chart to support only key: value type of env vars and not more complex use case, e.g. using valueFrom with a reference to a configmap.

[vanHavel]

That is true, I adapted my approach to also allow for complex env specifications, while still keeping the env var keys in a map.

However, #90 does also allow this (with lists), which is more directly following the k8s format of specifying env - I would be happy if either gets accepted.

[hagaibarel]

I think #90 addresses the env vars issue in a simpler way, let's focus on that one