Add stub implementation of private initialize method

Someone in the system calls this. Not yet clear who it so we're adding a stub
implementation which will log once it is called.

1970-01-14T03:25:44.731218Z [51] user.err keymanager [] luna-service LS_NO_METH
{"METHOD":"initialize"} Couldn't find method: initialize
1970-01-14T03:25:44.731554Z [51] user.err keymanager [] luna-service LS_NO_METH
{"METHOD":"initialize"} Couldn't find method: initialize

Signed-off-by: Simon Busch <morphis@gravedo.de>