Ansible role to deploy OSSEC on servers (manager) and clients (agents)

Motivation

OSSEC is a must-have on a web server and other machines connected to the Internet since it provides a host-based intrusion detection system.
Ansible is a fantastic tool to automate the deployment of software to different machines
I needed an easy and automatized way to deploy OSSEC to different machines while giving them some default configuration and still having the possibility to adjust the OSSEC configuration.

Ansible role should work for CentOS, Debian, RedHat, Scientific Linux and Ubuntu and is tested with Ansible version 2.1
OSSEC gets automatically installed on all machines (server / OSSEC term: manager and clients / OSSEC term: agents).
All clients / OSSEC term: agents obtain automatically a client key from the server and get a default OSSEC configuration
The configuration for each client can be individually adjusted.

Define all machines in the hosts file (both manager and clients).
Open the ossec.yml file and in the following section
```
authorized: [NameOfServer]
```
change NameOfServer to the DNS name of your manager machine, e.g. ossecserver.myorganization.net
Open the file roles/ossec/tasks/main.yml and replace all occurrences of NameOfServer with the name of your manager machine as before.

Note: If your manager machine is not using eth0 as its default ethernet port, you have to replace eth0 also by the proper interface name.
Note: Probably, one could define the manager DNS name once in Ansible, so that one would not have to replace the manager DNS name a few times. If someone knows how to do that I would be glad to get a PR.

If you would like to change the OSSEC default config or to add agent-specific rules, checkout the file roles/ossec/files/agent.conf and have a look at the dedicated manual page for further information about configuration options.

For the download of YUM packages the AtomiCorp repository and AGPL installation script is used as described in the OSSEC manual
The script to install the YUM package sources of AtomiCorp was slightly modified to allow an automatic installation of the sources.
Furthermore, it was extended, so that the RedHat packages are also getting installed on Scientific Linux.
The YUM package sources installation script can be found under roles/ossec/files/ossec-yum-repository