docs: Change quick install admin guide for debian

* Prepare quick install admin guide for the changes that will come with
the CentOS guide
docs: Add centos install guide

* Change the name of quick install admin guide to install guide debian
* Add CentOS installation guide (install-guide-centos)
* Update the doc index to comply with the aforementioned changes
* Update all pages that linked to the old guide
* Update both guides to be in sync with Synnefo 0.15
* Fix minor `bugs` in both Debian and CentOS install guides
* Add sections on how to create vlans/bridges/ssh-keys
* Update architecture images
common: Update default emails used in settings

* Update the default emails used in settings to point to example.com
instead of synnefo.org
cyclades: Minor bug fix in server create

* Fix a typo in logic/servers on server port creation
* Update the copyright date