Role postgresql: move all tasks to a 'centos' sub folder
The issue: Most of our tasks check the OS type and version. Those checks clutter the configuration files, making exploration and maintenance diffiult and unpleasant. The solution: The tasks were already separated by function, each group of task related to one function being in its own file. This commit will also group those files in folder dedicated to each type of OS. Note that this is the first step of the process.