Fail file_server template interpolation if no tls cert/key
When https server is enabled
[finishes #165872883](https://www.pivotaltracker.com/story/show/165872883)
log_level: p("diego.file_server.log_level"),
}
+ if_p("tls.cert") do |_|
config[:cert_file] = "#{conf_dir}/certs/tls.crt"
+ end
+
+ if_p("tls.key") do |_|
config[:key_file] = "#{conf_dir}/certs/tls.key"
+ end
+
+ if config[:https_server_enabled] && (config[:cert_file].nil? || config[:key_file].nil?)
+ raise "tls.cert and tls.key are required if https_server_enabled is set to true"
+ end
if_p("logging.format.timestamp") do |format|
if format != "unix-epoch" and format != "rfc3339"
