keytabs: Expose and modify key encoding function
    
    Make it available outside of the encoding.c file for use in a follow-up
    patch. Add option to not pass a password and generate a random key
    instead.
    
    Related:
    https://fedorahosted.org/freeipa/ticket/3859
    
    Reviewed-By: Nathaniel McCallum <npmccallum@redhat.com>