Identify Arch Linux as 'Arch' for consistency with Python 3 platform module

As of current Python 2.x, the platform module is not aware of Arch Linux. Appropriately, Ansible detects this distribution itself, mapping `/etc/arch-release` to a distribution identity of `Archlinux`.

In Python 3, the output of `platform.linux_distribution()` on Arch Linux is `('arch', 'Arch', 'Linux')`. Correspondingly, on this platform, the output of `lsb_release -s -i` (to get the distribution id in short form) is `Arch`.

For these reasons, the detection performed by the setup module should be normalized; otherwise, this will introduce an extra level of user-visible incompatibility (or need for mapping) on migration to Python 3.0.