Previously with client fingerprinting, sys/exec's Command
function was being used to execute `uname -r` and the return
string processed into the kernel.version node attribute.
This change uses gopsutil/host KernelVersion function
instead. This means we can drop the os/exec, strings and
fmt imports... and not execute an external binary.
Signed-off-by: Dave Walker (Daviey) <email@daviey.com>