collector: support custom poll intervals

Also simplify config context schema for SNMP since we only use a single
instance.

roles/collector/tasks/main.yml

@@ -28,7 +28,7 @@
   template:
     dest: "/etc/prometheus/conf.d/{{ item.name }}.yml"
     src: "prometheus-job.yml.j2"
-  loop: "{{ prometheus_hosts }}"
+  loop: "{{ prometheus_config }}"
   loop_control:
     label: "{{ item.name }}"
   notify: reload prometheus