Clean up container cache prep in tasks

This patch cleans up the container cache preparation in the tests
due to the following patches:

- https://review.openstack.org/307856 (new image build process)
- https://review.openstack.org/315114 (new apt config process)
- https://review.openstack.org/322188 (new resolver config process)

Change-Id: Iacc759b8bac213afc6cf8925036ad3d88feefc0e

tests/test-prepare-host.yml

@@ -35,16 +35,7 @@
       set_fact:
         lxc_container_cache_files:
           - { src: '/etc/pip.conf', dest: '/etc/pip.conf' }
-          - { src: '/etc/apt/apt.conf.d/99unauthenticated', dest: '/etc/apt/apt.conf.d/99unauthenticated' }
       when: nodepool.stat.exists | bool
-    - name: Determine the existing Ubuntu repo configuration
-      shell: 'awk "/^deb .*ubuntu\/? {{ ansible_distribution_release }} main/ {print \$2; exit}" /etc/apt/sources.list'
-      register: ubuntu_repo
-      changed_when: false
-    - name: Set apt repo facts based on discovered information
-      set_fact:
-        lxc_container_template_main_apt_repo: "{{ ubuntu_repo.stdout }}"
-        lxc_container_template_security_apt_rep: "{{ ubuntu_repo.stdout }}"
   post_tasks:
     - name: Create br-storage bridge
       shell: /sbin/brctl addbr br-storage || true