DCD-686: Install Postgres only if restoration is required.

roles/fetch_backups/tasks/amazon.yml

@@ -0,0 +1,7 @@
+---
+
+# Amazon Linux 2 supplies extra packages via a special command.
+- name: Enable Postgresql from 'extras'
+  command: amazon-linux-extras install -y "postgresql{{ postgres_version }}"
+  args:
+    creates: /usr/bin/psql

roles/fetch_backups/tasks/main.yml

@@ -69,4 +69,7 @@
         object: "{{ atl_backup_manifest.shared_home_dump | urlsplit('path') }}"
         dest: "{{ atl_backup_home_dest }}"
 
+    - name: Install distro-specific restore support packages
+      include_tasks: "{{ ansible_distribution|lower }}.yml"
+
   when: atl_backup_manifest_url is defined and atl_backup_manifest_url != ''