Update CLI to use ISO_8601 time format.

This change updates the formatTime CLI helper function to return
an ISO_8601 time format which will make CLI time usage more
consistent and easier. Previosuly the time format was in US style
format which was somewhat confusing to non US users.

Closes hashicorp#3806

command/helpers.go

@@ -59,7 +59,8 @@ func formatTime(t time.Time) string {
 		// It's more confusing to display the UNIX epoch or a zero value than nothing
 		return ""
 	}
-	return t.Format("01/02/06 15:04:05 MST")
+	// Return ISO_8601 time format GH-3806
+	return t.Format("2006-01-02T15:04:05Z07:00")
 }
 
 // formatUnixNanoTime is a helper for formatting time for output.