Set default docker_package for Archlinux distribution

[adamantike]

Instead of the current role's default value for docker_package (docker-ce), the package name for the Archlinux OS distribution is docker [1].

Considerations:

• Filename is Archlinux, as that is the possible value exposed by Ansible, in ansible_distribution [2] and ansible_os_family [3].
• Platform name in meta's galaxy_info has been changed to ArchLinux, as that's the supported platform name in Ansible Galaxy [4].

[1] https://archlinux.org/packages/community/x86_64/docker/ [2] https://docs.ansible.com/ansible/2.6/user_guide/playbooks_conditionals.html#ansible-distribution [3] https://docs.ansible.com/ansible/2.6/user_guide/playbooks_conditionals.html#ansible-os-family [4] https://galaxy.ansible.com/api/v1/platforms/?name__icontains=arch