Fix various unicode issues

Substitute ``synnefo.util.text.uenc`` occurrences
with the respective django encoding utilities
cyclades: Allow floating IPs from drained networks

This commit makes possible to create a port to a drained network when
using a floating IP that has been already allocated. The 'drained' flag
should only affect the allocation of new IPs and not the use of existing
ones.
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