Installation guide updates

- warn against using the native password based auth
- explicitly recommend configuring HTTPS
- cover using cobbler to download the bootloaders
- update core task addition instructions
- mention external services needed in each lab
- mention console server and archive server support
- mention firewall ports that need to be open
- explicitly note some future todo items

Bug: 1004622
Bug: 1032377
Change-Id: I3987683e0bce310cc0bbb2e4a101657b2949b960