distro/rhel7: add packageset chains to qcow2

The qcow2 image type for RHEL 7 doesn't have packageset chains defined.
This means that the blueprint packages are never merged into the os
pipeline.

This is unnecessary right now because of the upcoming rewrite, but it
will minimise the differences that will show up in the manifest.

The qcow2-customize manifest has an added block of options for the
grub2.legacy stage because now the dracut-config-rescue package is being
installed in the image.

internal/distro/rhel7/qcow2.go

@@ -102,6 +102,9 @@ var qcow2ImgType = imageType{
 		buildPkgsKey: distroBuildPackageSet,
 		osPkgsKey:    qcow2CommonPackageSet,
 	},
+	packageSetChains: map[string][]string{
+		osPkgsKey: {osPkgsKey, blueprintPkgsKey},
+	},
 	defaultImageConfig: &distro.ImageConfig{
 		DefaultTarget: common.ToPtr("multi-user.target"),
 		Sysconfig: []*osbuild.SysconfigStageOptions{