Implement timeouts for all exec command runners

[sparrc]

First is to write an internal CombinedOutput and Run function with a
timeout.

Second, the following instances of command runners need to have timeouts:

plugins/inputs/ping/ping.go
125:    out, err := c.CombinedOutput()

plugins/inputs/exec/exec.go
91: if err := cmd.Run(); err != nil {

plugins/inputs/ipmi_sensor/command.go
31: err := cmd.Run()

plugins/inputs/sysstat/sysstat.go
194:    out, err := cmd.CombinedOutput()

plugins/inputs/leofs/leofs.go
185:    defer cmd.Wait()

plugins/inputs/sysstat/sysstat.go
282:    if err := cmd.Wait(); err != nil {

closes #1067