Fix `in_cluster` authentication (#34)
Fix `in_cluster` authentication (#34)

This commit fixes the `in_cluster` authentication exposed by `Kazan.Server.in_cluster`:

- Changed the ca_cert file name to `ca.crt`.  This appears to be the filename used for the in cluster CA certificate from at least kubernetes 1.6.  See #25 for more details.
- Use environment variables to get the k8s server name & port, rather than hardcoding to the DNS name `kubernetes`.

Resolves #25 
obmarg/kazan