pkg/parsers/operatingsystem: don't use strings.SplitN()

We're looking for a specific prefix, so remove the prefix instead. Also remove
redundant error-wrapping, as `os.Open()` already provides details in the error
returned;

    open /no/such/file: no such file or directory
    open /etc/os-release: permission denied

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/parsers/operatingsystem/operatingsystem_linux.go

@@ -5,7 +5,6 @@ package operatingsystem // import "github.com/docker/docker/pkg/parsers/operatin
 import (
 	"bufio"
 	"bytes"
-	"fmt"
 	"os"
 	"strings"
 )
@@ -44,23 +43,22 @@ func getValueFromOsRelease(key string) (string, error) {
 	osReleaseFile, err := os.Open(etcOsRelease)
 	if err != nil {
 		if !os.IsNotExist(err) {
-			return "", fmt.Errorf("Error opening %s: %v", etcOsRelease, err)
+			return "", err
 		}
 		osReleaseFile, err = os.Open(altOsRelease)
 		if err != nil {
-			return "", fmt.Errorf("Error opening %s: %v", altOsRelease, err)
+			return "", err
 		}
 	}
 	defer osReleaseFile.Close()
 
 	var value string
-	keyWithTrailingEqual := key + "="
 	scanner := bufio.NewScanner(osReleaseFile)
 	for scanner.Scan() {
 		line := scanner.Text()
-		if strings.HasPrefix(line, keyWithTrailingEqual) {
-			data := strings.SplitN(line, "=", 2)
-			value = strings.Trim(data[1], `"' `) // remove leading/trailing quotes and whitespace
+		if strings.HasPrefix(line, key+"=") {
+			value = strings.TrimPrefix(line, key+"=")
+			value = strings.Trim(value, `"' `) // remove leading/trailing quotes and whitespace
 		}
 	}