Add environment variables on container specs

[edwarddoh]

It would be great to be able to add environment variables for coredns container.
Our usecase is that we enable role base authentications for the etcd, and to provide a credential information on the etcd plugins for that, we make our coredns configuration block to use environment variables.

our value file is looking like,

...
   plugins:
   - name: etcd
     configBlock: |-
        path /skydns/corp
        endpoint http://10.10.0.123:2379
        credentials {$ETCD_USERNAME} {$ETCD_PASSWORD}
...

To be able to use environment variable that is on the config file, we would like to make this change so that we can pass it through secrets. (one thing we would like to avoid the most is to put the password in the value file..)

[hagaibarel]

Definitely something we should support, please rebase and bump the chart version by a minor version