add simple preflight checks

tasks/preflight.yml

@@ -1 +1,10 @@
 ---
+- name: Assert usage of systemd as an init system
+  assert:
+    that: ansible_service_mgr == 'systemd'
+    msg: "This role only works with systemd"
+
+- name: Naive assertion of proper listen address
+  assert:
+    that:
+      - "':' in blackbox_exporter_web_listen_address"