Add string_rand() function

`string_rand()` behaves mostly like `fqdn_rand(`). The difference being the
`fqdn` is not used as a seed to generate a random number, rather a string
(or interger) argument is used.

Since `fqdn_rand()` provides a random number for a single host, this function can be useful in providing a random number that can be used as an identifier for clustered services (across multiple hosts).