Document and Increase server installation automation

A part of the automation is already done in the cea infra:

• DHCP
• PXE boot

It "only" misses the installation part to easily add a server in the infra. A basic installation via a preseed file should be possible.

Document how to:

• Pre installation tasks (switch ports/hardware raid configuration/...)
• Generate a preseed file for a server
• Install this server from the preseed
• Post installation tasks (osd raid volumes/ansible/...)