First of all, it's important to recognize that "Reformed theology" does not mean "theology of the reformers" – the terminology is a bit confusing, which is why the tradition is often referred to as Calvinism, even though Calvin was just one of many theologians in this tradition. Other commonly cited "Reformed" confessions are the Belgic Confession and the 1689 Baptist Confession.
We can find the answer to your question in the Westminster Confession itself, chapter 7, "Of God's Covenant with Man." Paragraphs 2 and 3 describe the "covenant of works" (or "covenant of life") that was made with Adam, and the "covenant of grace," which God instituted following Adam's failure to keep the first covenant.
The confession describes the covenant of grace as follows:
The Lord was pleased to make a second, commonly called the covenant of grace; wherein He freely offers unto sinners life and salvation by Jesus Christ; requiring of them faith in Him, that they may be saved, and promising to give unto all those that are ordained unto eternal life His Holy Spirit, to make them willing, and able to believe. (§3)
Some might assume that the reference to Jesus Christ means that this covenant only went into effect after Jesus's sacrifice, but that's not what is meant. Instead, the covenant of grace has been in place ever since Adam's fall, but has manifested itself in different ways:
This covenant was differently administered in the time of the law, and in the time of the Gospel: under the law it was administered by promises, prophecies, sacrifices, circumcision, the paschal lamb, and other types and ordinances delivered to the people of the Jews, all foresignifying Christ to come; which were, for that time, sufficient and efficacious, through the operation of the Spirit, to instruct and build up the elect in faith in the promised Messiah, by whom they had full remission of sins, and eternal salvation; and is called the Old Testament. (§5)
So "the time of the law" (the Old Testament) is still part of the "covenant of grace," and God's people of that time were saved by Jesus, even though he had not yet come. But the manifestation of the covenant during that time was substantially different from "the time of the Gospel":
Under the Gospel, when Christ, the substance, was exhibited, the ordinances in which this covenant is dispensed are the preaching of the Word, and the administration of the sacraments of Baptism and the Lord's Supper: which, though fewer in number, and administered with more simplicity, and less outward glory, yet, in them, it is held forth in more fullness, evidence, and spiritual efficacy, to all nations, both Jews and Gentiles; and is called the New Testament. There are not therefore two covenants of grace, differing in substance, but one and the same, under various dispensations. (§6)
Luther's view of all this is a matter of some debate. It's generally accepted, however, that he spoke less of the idea of "covenant" than Reformed theologians did, and that he focused more on a Law/Gospel distinction that was more closely associated with the division between the Old and New Testaments (or between Moses and Christ), not between a "covenant of works" and "covenant of grace."
These distinctions are seen in Chapter 5 of the Formula of Concord, written by Lutherans a generation after Luther:
We believe, teach, and confess that the distinction between the Law and the Gospel is to be maintained in the Church with great diligence as an especially brilliant light, by which, according to the admonition of St. Paul, the Word of God is rightly divided. (§1)
Everything that reproves sin is, and belongs to, the preaching of the Law. (§3)
The Gospel is properly such a doctrine as teaches what man who has not observed the Law, and therefore is condemned by it, is to believe, namely, that Christ has expiated and made satisfaction for all sins, and has obtained and acquired for him, without any merit of his [no merit of the sinner intervening], forgiveness of sins, righteousness that avails before God, and eternal life. (§4)
There are certainly significant differences between the Lutheran and Reformed views of the covenant, but some have argued that the Lutheran Law/Gospel distinction is more a matter of differing emphasis than major disagreement. Still, these quotes make it clear that there are differences in how the two traditions approach the Law and the Old Testament.