Implement timeouts for all exec command runners

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

plugins/inputs/ipmi_sensor/command.go

@@ -1,10 +1,12 @@
 package ipmi_sensor
 
 import (
-	"bytes"
 	"fmt"
 	"os/exec"
 	"strings"
+	"time"
+
+	"github.com/influxdata/telegraf/internal"
 )
 
 type CommandRunner struct{}
@@ -18,21 +20,16 @@ func (t CommandRunner) cmd(conn *Connection, args ...string) *exec.Cmd {
 	}
 
 	return exec.Command(path, opts...)
-
 }
 
 func (t CommandRunner) Run(conn *Connection, args ...string) (string, error) {
 	cmd := t.cmd(conn, args...)
-	var stdout bytes.Buffer
-	var stderr bytes.Buffer
-	cmd.Stdout = &stdout
-	cmd.Stderr = &stderr
 
-	err := cmd.Run()
+	output, err := internal.CombinedOutputTimeout(cmd, time.Second*5)
 	if err != nil {
 		return "", fmt.Errorf("run %s %s: %s (%s)",
-			cmd.Path, strings.Join(cmd.Args, " "), stderr.String(), err)
+			cmd.Path, strings.Join(cmd.Args, " "), string(output), err)
 	}
 
-	return stdout.String(), err
+	return string(output), err
 }