Provide a way to resolve configuration value from environment

[yuinacor]

In k8s environment, hostname decided when pod is created.

To use current saml authentication configuration, we substitute some environment variables in dogma.json before centraldogma starts. Hostname is just that case.

It seems happy to provide some way to resolve configuration properties from environment variables like spring boot configuration properties.

It's blocker issue to resolve https://github.com/line/centraldogma/issues/1006.