Remove ECS policies from AWS compute policy

The compute policy was exceeding maximum size and contained
policies that already exist in ecs-policy.

Look up suitable AMIs rather than hardcode

We don't want to maintain multiple image IDs for multiple regions
so use ec2_ami_facts to set a suitable image ID

Improve exception handling

test/integration/targets/ecs_cluster/playbooks/roles/ecs_cluster/defaults/main.yml

@@ -1,11 +1,3 @@
-# http://docs.aws.amazon.com/AmazonECS/latest/developerguide/ecs-optimized_AMI.html
-# amzn-ami-2017.09.b-amazon-ecs-optimized
-ecs_agent_images:
-  us-east-1: ami-71ef560b
-  us-east-2: ami-1b8ca37e
-  us-west-2: ami-d2f489aa
-  us-west-1: ami-6b81980b
-
 ecs_cluster_name: "{{ resource_prefix }}"
 user_data: |
   #!/bin/bash

test/integration/targets/ecs_cluster/playbooks/roles/ecs_cluster/tasks/main.yml

@@ -123,12 +123,24 @@
         <<: *aws_connection_info
       register: setup_sg
 
+    - name: find a suitable AMI
+      ec2_ami_facts:
+        owner: amazon
+        filters:
+          description: "Amazon Linux AMI* ECS *"
+        <<: *aws_connection_info
+      register: ec2_ami_facts
+
+    - name: set image id fact
+      set_fact:
+        ecs_image_id: "{{ (ec2_ami_facts.images|first).image_id }}"
+
     - name: provision ec2 instance to create an image
       ec2:
         key_name: '{{ ec2_keypair|default(setup_key.key.name) }}'
         instance_type: t2.micro
         state: present
-        image: '{{ ecs_agent_images[aws_region] }}'
+        image: '{{ ecs_image_id }}'
         wait: yes
         user_data: "{{ user_data }}"
         instance_profile_name: ecsInstanceRole