Confirm local debs are present for upgrade scenario

If the upgrade scenario is used with the local debs strategy (i.e.,
QA_APT_TEST env var is not set), we should fail fast without the
required local deb files. Previously, the scenario would proceed all the
way to the local apt server configuration and only fail when attempting
to sign the Release file with a test key.

Now, the role will fail almost immediately without local debs present.
Hat tip to @zenmonkeykstop for flagging during review.

molecule/upgrade/playbook.yml

@@ -1,4 +1,30 @@
 ---
+- name: Confirm local debs present if required
+  hosts: localhost
+  gather_facts: no
+  vars:
+    molecule_dir: "{{ lookup('env', 'MOLECULE_SCENARIO_DIRECTORY') }}"
+  tasks:
+      # Before running apt-server logic, confirm we have local
+      # debs built.
+    - name: Establish QA strategy
+      set_fact:
+        QA_APTTEST: "{{ lookup('env','QA_APTTEST')|bool }}"
+
+    - name: Find local deb files
+      find:
+        paths: "{{ molecule_dir }}/../../build/"
+        patterns: "*.deb"
+      register: _upgrade_scenario_find_debs_result
+      when: not QA_APTTEST
+
+    - name: Ensure debs were found
+      assert:
+        that:
+          - "_upgrade_scenario_find_debs_result.files|length >= 8"
+        msg: "No local debs found, run 'make build-debs'"
+      when: not QA_APTTEST
+
 - name: Re-run original tor role
   hosts: securedrop
   max_fail_percentage: 0