Sign rpm repo metadata (gravitational#9027)

This helps support zypper on Suse, and improves our general RPM
distribution security posture.  The threat model is someone compromises
AWS, but not our signing keys.  In this case, they could update repo
metatdata to point to an unsigned package.  With metadata signed, this
is no longer possible -- both the index and the package are verified.

For more info on this change, see this very helpful blog post:

  https://blog.packagecloud.io/eng/2014/11/24/howto-gpg-sign-verify-rpm-packages-yum-repositories/

.drone.yml

@@ -5016,6 +5016,31 @@ steps:
     - yum -y install createrepo
     - createrepo --cachedir /rpmrepo/teleport/cache --update /rpmrepo/teleport
 
+  # This step requires centos:8 to get gpg 2.2+
+  # centos:7's gpg 2.0 doesn't understand the format of GPG_RPM_SIGNING_ARCHIVE
+  - name: Sign RPM repo metadata
+    image: centos:8
+    volumes:
+      - name: rpmrepo
+        path: /rpmrepo
+      # for in-memory tmpfs for key material
+      - name: tmpfs
+        path: /tmpfs
+    environment:
+      GNUPGHOME: /tmpfs/gnupg
+      GPG_RPM_SIGNING_ARCHIVE:
+        from_secret: GPG_RPM_SIGNING_ARCHIVE
+    commands:
+      - |
+        # extract signing key
+        mkdir -m0700 $GNUPGHOME
+        echo "$GPG_RPM_SIGNING_ARCHIVE" | base64 -d | tar -xzf - -C $GNUPGHOME
+        chown -R root:root $GNUPGHOME
+      # Sign rpm repo metadata (yum clients will automatically look for and verify repodata/repomd.xml.asc)
+      - gpg --detach-sign --armor /rpmrepo/teleport/repodata/repomd.xml
+      - cat /rpmrepo/teleport/repodata/repomd.xml.asc
+      - rm -rf $GNUPGHOME
+
   - name: Sync RPM repo changes to S3
     image: amazon/aws-cli
     environment:
@@ -5141,6 +5166,6 @@ volumes:
       name: drone-s3-debrepo-pvc
 ---
 kind: signature
-hmac: 5024d8ffe4db0d734fcb1f1a2a22d3ae2078d270dbaac7900eca0db2d1448655
+hmac: 1473746cd33150de6ce4e6be53478ad6961414d1e34987d9eff4b3e17bcfe5a2
 
 ...