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>

docs/sources/reference/api/docker_remote_api_v1.18.md

@@ -1452,6 +1452,7 @@ Display system-wide information
              "Debug":false,
              "NFd": 11,
              "NGoroutines":21,
+             "SystemTime": "2015-03-10T11:11:23.730591467-07:00"
              "NEventsListener":0,
              "InitPath":"/usr/bin/docker",
              "InitSha1":"",