Update install guide, add CentOS installation guide, update default values for email settings and fix a minor bug

* Add CentOS installation guide
* Update installation guides to comply with version 0.15 of Synnefo
* Add sections on installation guides on how to create vlans/bridges/ssh-keys needed for Ganeti
* Fix typos in installation guides
* Update all pages that linked to the Debian Installation Guide, to also link to CentOS Installation Guide
* Update emails used in settings to point to example.com instead of synnefo.org
* Fix a bug (typo) on server port creation