Define and implement specs for bootstrap-os (#4455)

* Add README to bootstrap-os role

* Rework bootstrap-os once more

* Document workarounds for bugs/deficiencies in Ansible modules
* Unify and document role variables
* Remove installation of additional packages and repositories
* Merge Ubuntu and Debian tasks
* Remove pipelining setting from default playbooks
* Fix OpenSUSE not running its required tasks

roles/bootstrap-os/README.md

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+# bootstrap-os
+
+Bootstrap an Ansible host to be able to run Ansible modules.
+
+This role will:
+  * configure the package manager (if applicable) to be able to fetch packages
+  * install Python
+  * install the necessary packages to use Ansible's package manager modules
+  * set the hostname of the host to `{{ inventory_hostname }}` when requested
+
+## Requirements
+
+A host running an operating system that is supported by Kubespray.
+See https://github.com/kubernetes-sigs/kubespray#supported-linux-distributions for a current list.
+
+SSH access to the host.
+
+## Role Variables
+
+Variables are listed with their default values, if applicable.
+
+### General variables
+
+  * `http_proxy`/`https_proxy`
+    The role will configure the package manager (if applicable) to download packages via a proxy.
+    This is currently implemented for CentOS/RHEL (`http_proxy` only) as well as Debian and Ubuntu (both `http_proxy` and `https_proxy` are respected)
+
+  * `override_system_hostname: true`
+    The role will set the hostname of the machine to the name it has according to Ansible's inventory (the variable `{{ inventory_hostname }}`).
+
+### Per distribution variables
+
+#### CoreOS
+
+* `coreos_locksmithd_disable: false`
+  Whether `locksmithd` (responsible for rolling restarts) should be disabled or be left alone.
+
+#### CentOS/RHEL
+
+* `centos_fastestmirror_enabled: false`
+  Whether the [fastestmirror](https://wiki.centos.org/PackageManagement/Yum/FastestMirror) yum plugin should be enabled.
+
+## Dependencies
+
+The `kubespray-defaults` role is expected to be run before this role.
+
+## Example Playbook
+
+Remember to disable fact gathering since Python might not be present on hosts.
+
+    - hosts: all
+      gather_facts: false  # not all hosts might be able to run modules yet
+      roles:
+         - kubespray-defaults
+         - bootstrap-os
+
+## License
+
+Apache 2.0