Check /etc/hostname to derive minion id

This pull request fixes #6645 by making salt look at /etc/hostname first before running ``socket.getfqdn()``. This fixes ID generation for distros like Arch Linux which respect the Linux FHS and require that the hostname be placed in /etc/hostname.

In addition, this pull request also adds documentation on how the minion id is derived to the walkthrough, and also x-refs the minion config file documentation for the ``id`` parameter.