Add system time to /info

This change adds daemon's system time as RFC3339Nano to the `/info` endpoint
and shows in a more readable format (UnixDate) in `docker -D info` output.

I will be using this to fix the clock skew between the remote test host and
the CI machines running `docker events`-related tests as they're using `--since`
and `--until` and the timestamps are not matching when daemon is not on the
same machine.

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

engine/env.go

@@ -7,6 +7,7 @@ import (
 	"io"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/docker/docker/utils"
 )
@@ -69,6 +70,15 @@ func (env *Env) SetBool(key string, value bool) {
 	}
 }
 
+func (env *Env) GetTime(key string) (time.Time, error) {
+	t, err := time.Parse(time.RFC3339Nano, env.Get(key))
+	return t, err
+}
+
+func (env *Env) SetTime(key string, t time.Time) {
+	env.Set(key, t.Format(time.RFC3339Nano))
+}
+
 func (env *Env) GetInt(key string) int {
 	return int(env.GetInt64(key))
 }