traffic_replay: Generate machine accounts as well as users

traffic_replay: Generate machine accounts as well as users

Currently the tool only generates the machine accounts needed for
traffic generation. However, this isn't realistic if we're trying to use
the tool to generate users to simulate a large network.

This patch generates machine accoutns along with the user accounts.
Note we assume there will be more computer accounts than users in a real
network (e.g. work laptops, servers, etc), so generate slightly more
computer accounts.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>