Refactor out userLookup to helper package

Also replaces user.Lookup in exec driver

client/allocdir/alloc_dir_posix.go

@@ -5,11 +5,10 @@ package allocdir
 
 import (
 	"fmt"
-	"io/ioutil"
+	"github.com/hashicorp/nomad/helper/user-lookup"
 	"os"
 	"os/user"
 	"strconv"
-	"strings"
 	"syscall"
 )
 
@@ -28,7 +27,7 @@ func (d *AllocDir) dropDirPermissions(path string) error {
 		return nil
 	}
 
-	u, err := userLookup("nobody")
+	u, err := userlookup.Lookup("nobody")
 	if err != nil {
 		return err
 	}
@@ -71,28 +70,3 @@ func getGid(u *user.User) (int, error) {
 
 	return gid, nil
 }
-
-// userLookup checks if the given username or uid is present in /etc/passwd
-// and returns the user struct.
-// If the username is not found, an error is returned.
-// Credit to @creak, https://github.com/docker/docker/pull/1096
-func userLookup(uid string) (*user.User, error) {
-	file, err := ioutil.ReadFile("/etc/passwd")
-	if err != nil {
-		return nil, err
-	}
-
-	for _, line := range strings.Split(string(file), "\n") {
-		data := strings.Split(line, ":")
-		if len(data) > 5 && (data[0] == uid || data[2] == uid) {
-			return &user.User{
-				Uid:      data[2],
-				Gid:      data[3],
-				Username: data[0],
-				Name:     data[4],
-				HomeDir:  data[5],
-			}, nil
-		}
-	}
-	return nil, fmt.Errorf("User not found in /etc/passwd")
-}