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