* Refactor of kubeadm images listing

Instead of setting multiples facts, we directly create the dict we need from
kubeadm output.

* Remove useless 'default' filters in roles/download

* Only download kubeadm images where needed

* Remove checks for docs using exact tags

Instead use a more generic documentation for installing kubespray as a
collection from git.

* Check that we upgraded galaxy.yml to next version

This is only intented to check for human error. The version in galaxy
should be the next (which does not mean the same if we're on master or a
release branch).

* Set collection version to KUBESPRAY_NEXT_VERSION

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>