Separate irods tasks by platform

See merge request !4

Any task that is not OS-specific is moved to the 'common' folder.

Remove anything not Debian-specific from the 'debian' folder.

Remove anything not Ubuntu-specific in the 'ubuntu' folder.

Remove anything that is not CentOS-specific.

Step one of a refactoring process where we are going to split the tasks by
target OS.

Separate postgres tasks by platform

See merge request !3

The conditions were all written on a single line which makes it hard to quickly
identify what they are.

This commit separates then each on their own line, which is easier to read.

Caveat: One must be aware that all conditions of all lines must be statisfied;
it's and 'AND' and not an 'OR'.

All tasks that are OS-agnostic are moved to the 'common' folder.

Remove anything that is not Debian-specific in the 'debian' folder.

Remove anything not Ubuntu-specific in the 'unbuntu' directory.

Remove anything that is not CentOS-specific in the 'centos' folder.

Add a task import for the tasks/centos/main.yml file in the tasks/main.yml
file.
With this we effectively have factorized all CentOS distribution checks into
only one check.

Duplicate the tasks so that we can, in next commits, eliminate the unrelated
tasks for each OS.

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.

Role postgresql, task systemd: factorize tasks.

See merge request !2

This is an improvement to the systemd.yml file which had a lot
of tasks doing the same thing, with slighty different names for the
unit files.

I factorized the 'start postgresql.service' tasks so that we only need
one task with a dynamic unit file name.

Remarks: This means that we no longer have system-specific tasks.
It doesn't matter in the current state as there was nothing system-specific
in our tasks.

Postgresql systemd improvement

See merge request !1

The issue:
The 'postgresql_version' variable is only collected for Ubuntu 18 platforms.
This caused the label of the task to bug out and display weird information.

The solution:
Since it's only the label and we don't need to be 100% accurate, I used the
'external_postgresql_version' which is user-defined.
No more bugs. Nice label.

The issue:
In its previous state the task directly edited postgresql's systemd unit file.
This file is systematically replaced by each postgresql update.
This is fine as long as we roll the update with this role only, but will break
as soon as an update is rolled by any other mean.

The solution:
Create an override file located in /etc/systemd/system/postgresql.service with
the appropriate custom options for the unit file (what we used to write in the
unit file).
This file will not be overriden and is therefore more stable.

Added new use_distribution_postgresql variable to README.md. Also renamed postgresql_version variable to external_postgresql_version