Record artifact checksums and signatures to stdout

In case of later upload failures, record the SHA2-256 checksum and
ASCII-armored OpenPGP signature of each signed artifact to the job's
output stream so they can later be used for manual uploading.

Change-Id: Ifd136b95357d499e088c5509fa57daf76a246cf4

roles/sign-artifacts/tasks/main.yaml

@@ -38,3 +38,17 @@
   file:
     path: "{{ gnupg_tmpdir.path }}"
     state: absent
+
+- name: Record checksums
+  command: "sha256sum {{ zj_artifact.path }}"
+  with_items: "{{ artifacts.files }}"
+  loop_control:
+    loop_var: zj_artifact
+  when: artifacts.matched > 0
+
+- name: Record signatures
+  command: "cat {{ zj_artifact.path }}.asc"
+  with_items: "{{ artifacts.files }}"
+  loop_control:
+    loop_var: zj_artifact
+  when: artifacts.matched > 0