Fix #296 (Cyberghost implementation) (#297)

* Reads the client key from /gluetun/client.key
* Read the client certificate from /gluetun/client.crt
* Additional checks for client key and client certificate validity
* Fix client key file parsing if environment variable isn't present

internal/cli/cli.go

@@ -21,7 +21,7 @@ import (
 
 func ClientKey(args []string) error {
 	flagSet := flag.NewFlagSet("clientkey", flag.ExitOnError)
-	filepath := flagSet.String("path", "/files/client.key", "file path to the client.key file")
+	filepath := flagSet.String("path", string(constants.ClientKey), "file path to the client.key file")
 	if err := flagSet.Parse(args); err != nil {
 		return err
 	}