Add support for releasever variable in signing key field for repository definitions

[Conan-Kudo]

Problem description

Fedora Linux repository definitions use a different GPG signing key for every release. The key is named with the release version in it to match the Fedora release, but while we can specify $releasever/${releasever} for the repository URL because it gets handled directly by DNF, we cannot do the same for the signing key attribute because kiwi does not translate this from the release-version preference setting in the description.

Please add support for this so that supporting Fedora releases is less painful.

Expected behavior

When release-version changes, the repository definition (with the repository URL and the signing key) is handled properly and kiwi imports the correct signing key.

Steps to reproduce the behavior

Add the following snippet to your kiwi description:

<image>
	<repository type="rpm-md" alias="fedora" sourcetype="metalink">
		<source path="https://mirrors.fedoraproject.org/metalink?repo=fedora-$releasever&amp;arch=$basearch">
			<signing key="file:///usr/share/distribution-gpg-keys/fedora/RPM-GPG-KEY-fedora-$releasever-primary"/>
		</source>
	</repository>
        <repository type="rpm-md" alias="updates" sourcetype="metalink">
                <source path="https://mirrors.fedoraproject.org/metalink?repo=updates-released-f$releasever&amp;arch=$basearch">
                        <signing key="file:///usr/share/distribution-gpg-keys/fedora/RPM-GPG-KEY-fedora-$releasever-primary"/>
                </source>
        </repository>
</image>

And have the release-version preference set to some Fedora release (like 39).

The GPG key import will fail, because $releasever does not get replaced with the value of release-version.

OS and Software information

• KIWI version: 9.25.13
• Operating system host version: Fedora Linux 39
• Operating system target version: Fedora Linux 39
• Open Build Service version (N/A if not using OBS): N/A
• Koji version (N/A if not using Koji): N/A

[Conan-Kudo]

cc: @davide125, @davdunc, @xevious