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/exec/exec.go

@@ -6,10 +6,12 @@ import (
 	"os/exec"
 	"sync"
 	"syscall"
+	"time"
 
 	"github.com/gonuts/go-shellquote"
 
 	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/internal"
 	"github.com/influxdata/telegraf/plugins/inputs"
 	"github.com/influxdata/telegraf/plugins/parsers"
 	"github.com/influxdata/telegraf/plugins/parsers/nagios"
@@ -19,6 +21,9 @@ const sampleConfig = `
   ## Commands array
   commands = ["/tmp/test.sh", "/usr/bin/mycollector --foo=bar"]
 
+  ## Timeout for each command to complete.
+  timeout = "5s"
+
   ## measurement name suffix (for separating different commands)
   name_suffix = "_mycollector"
 
@@ -32,6 +37,7 @@ const sampleConfig = `
 type Exec struct {
 	Commands []string
 	Command  string
+	Timeout  internal.Duration
 
 	parser parsers.Parser
 
@@ -43,7 +49,8 @@ type Exec struct {
 
 func NewExec() *Exec {
 	return &Exec{
-		runner: CommandRunner{},
+		runner:  CommandRunner{},
+		Timeout: internal.Duration{Duration: time.Second * 5},
 	}
 }
 
@@ -73,7 +80,11 @@ func AddNagiosState(exitCode error, acc telegraf.Accumulator) error {
 	return nil
 }
 
-func (c CommandRunner) Run(e *Exec, command string, acc telegraf.Accumulator) ([]byte, error) {
+func (c CommandRunner) Run(
+	e *Exec,
+	command string,
+	acc telegraf.Accumulator,
+) ([]byte, error) {
 	split_cmd, err := shellquote.Split(command)
 	if err != nil || len(split_cmd) == 0 {
 		return nil, fmt.Errorf("exec: unable to parse command, %s", err)
@@ -84,7 +95,7 @@ func (c CommandRunner) Run(e *Exec, command string, acc telegraf.Accumulator) ([
 	var out bytes.Buffer
 	cmd.Stdout = &out
 
-	if err := cmd.Run(); err != nil {
+	if err := internal.RunTimeout(cmd, e.Timeout.Duration); err != nil {
 		switch e.parser.(type) {
 		case *nagios.NagiosParser:
 			AddNagiosState(err, acc)