Elytron Client configuration file requires alias for keystore

In Elytron Client configuration file all elements related to keystore (key-store-ssl-certificate, key-store-reference and key-store-credential) require element alias. It is difference from Elytron subsystem where alias is not required.

Usage of alias should be consistent between subsystem and Elytron Client configuration file. Elytron is able to found out certificate without passed alias if keystore includes just one entry.