(#15791) isolate deterministic random function

We should not fiddle with the global seed, as other people
might still depend on Kernel.rand() not being that deterministic
as it is the idea with the fqdn_rand function.

However, as ruby < 1.9.2 does not provide such a nice solution, as
a dedicated Random class we simply call Kernel.srand() again on ruby
< 1.9.2 -> Reinitialize with a less deterministic seed.