stages/dnf: only write known options to repo file

Don't pass through arbitrary options. This means that pipeline repo
objects don't have the same options as dnf repo files anymore:

1. Hard code repo name to repo id. The name has no influence on the
resulting image and should thus not appear in a pipeline.

2. Set gpgcheck=1 when gpgkey is given. It defaults to false, which
means that all sample and test pipelines didn't verify packages. It
would have failed anyway, because the container doesn't have the key
referenced in /etc. Change all gpgkeys to refer to the key id and import
them manually.

3. Don't allow lists for baseurl and gpgkey. We can add that if we need
it at some point.

samples/base.json

@@ -8,9 +8,8 @@
         "basearch": "x86_64",
         "repos": {
           "fedora": {
-            "name": "Fedora",
             "metalink": "https://mirrors.fedoraproject.org/metalink?repo=fedora-$releasever&arch=$basearch",
-            "gpgkey": "file:///etc/pki/rpm-gpg/RPM-GPG-KEY-fedora-$releasever-$basearch"
+            "gpgkey": "F1D8 EC98 F241 AAF2 0DF6  9420 EF3C 111F CFC6 59B9"
           }
         },
         "packages": [