Added file and container image caching (#4828)

* File and container image downloads are now cached localy, so that repeated vagrant up/down runs do not trigger downloading of those files. This is especially useful on laptops with kubernetes runnig locally on vm's. The total size of the cache, after an ansible run, is currently around 800MB, so bandwidth (=time) savings can be quite significant.

* When download_run_once is false, the default is still not to cache, but setting download_force_cache will still enable caching.

* The local cache location can be set with download_cache_dir and defaults to /tmp/kubernetes_cache

* A local docker instance is no longer required to cache docker images; Images are cached to file. A local docker instance is still required, though, if you wish to download images on localhost.

* Fixed a FIXME, wher the argument was that delegate_to doesn't play nice with omit. That is a correct observation and the fix is to use default(inventory_host) instead of default(omit). See ansible/ansible#26009

* Removed "Register docker images info" task from download_container and set_docker_image_facts because it was faulty and unused.

* Removed redundant when:download.{container,enabled,run_once} conditions from {sync,download}_container.yml

* All features of commit d6fd0d2aca by Timoses <timosesu@gmail.com>, merged May 1st 2019, are included in this patch. Not all code was included verbatim, but each feature of that commit was checked to be working in this patch. One notable change: The actual downloading of the kubeadm images was moved to {download,sync)_container, to enable caching.

Note 1: I considered splitting this patch, but most changes that are not directly related to caching, are a pleasant by-product of implementing the caching code, so splitting would be impractical.

Note 2: I have my doubts about the usefulness of the upload, download and upgrade tags in the download role. Must they remain or can they be removed? If anybody knows, then please speak up.

roles/download/tasks/set_docker_image_facts.yml

@@ -1,50 +0,0 @@
----
-- name: Set if containers should be pulled by digest
-  set_fact:
-    pull_by_digest: >-
-      {%- if download.sha256 is defined and download.sha256 -%}true{%- else -%}false{%- endif -%}
-
-- name: Set pull_args
-  set_fact:
-    pull_args: >-
-      {%- if pull_by_digest %}{{ download.repo }}@sha256:{{ download.sha256 }}{%- else -%}{{ download.repo }}:{{ download.tag }}{%- endif -%}
-
-- name: Register docker images info
-  shell: >-
-    {{ docker_bin_dir }}/docker images -q | xargs -r {{ docker_bin_dir }}/docker inspect -f "{{ '{{' }} if .RepoTags {{ '}}' }}{{ '{{' }} (index .RepoTags) {{ '}}' }}{{ '{{' }} end {{ '}}' }}{{ '{{' }} if .RepoDigests {{ '}}' }},{{ '{{' }} (index .RepoDigests) {{ '}}' }}{{ '{{' }} end {{ '}}' }}" | sed -e 's/^ *\[//g' -e 's/\] *$//g' -e 's/ /\n/g' | tr '\n' ','
-  no_log: true
-  register: docker_images
-  failed_when: false
-  changed_when: false
-  check_mode: no
-  when:
-    - not download_always_pull
-    - group_names | intersect(download.groups) | length
-
-- name: Set if pull is required per container
-  set_fact:
-    pull_required: >-
-      {%- if pull_args in docker_images.stdout.split(',') %}false{%- else -%}true{%- endif -%}
-  when:
-    - not download_always_pull
-    - group_names | intersect(download.groups) | length
-
-- name: Does any host require container pull?
-  vars:
-    hosts_pull_required: "{{ hostvars.values() | map(attribute='pull_required') | select('defined') | list }}"
-  set_fact:
-    any_pull_required: "{{ True in hosts_pull_required }}"
-  run_once: true
-  changed_when: false
-  when: not download_always_pull
-
-- name: Check the local digest sha256 corresponds to the given image tag
-  assert:
-    that: "{{ download.repo }}:{{ download.tag }} in docker_images.stdout.split(',')"
-  when:
-    - group_names | intersect(download.groups) | length
-    - not download_always_pull
-    - not pull_required
-    - pull_by_digest
-  tags:
-    - asserts