Add a `KeyPassphrase` option to the admin configuration

Currently, the REST interface of the gateway can be started in HTTPS mode by giving a certificate & a private key files in the admin configuration

Most of the time, when generating a key/certificate pair, the key is encrypted with a passphrase. Currently, the gateway does not implement a mechanism to decrypt the key when it is. Thus, an option should be added to the admin configuration for the key passphrase, to allow the gateway to decrypt the key if it is encrypted.