Without a seed, the "random" script path locations for the remote-exec
provisioner were actually deterministic!
Every rand.Int31() would return the same pseudorandom chain starting w/
the numbers: 1298498081, 2019727887, 1427131847, 939984059, ...
So here we properly seed the communicators so the script paths are
actually random, and multiple runs on a single remote host have much
less chance of clobbering each other.
Fixes#4186
Kudos to @DustinChaloupka for the correct hunch leading to this fix!
96c20f0dd7