core: ticket utils: use SecureRandom

I had a glance at java implementation of basic Random, and it is bad for
us to generate the salt. So I suggest to use SecureRandom instead.

Change-Id: Idaaf8b9cc4c4afeb62d5ad5ba86fcb4878de09a8
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>