v2.7.2: Fix SELinux behavior and tidy up installation (#156)

* moved SELinux related stuff to own file

- conditionally included only when SELinux enabled, fixes #28
- unification of the way how Python SELinux libs are installed on RedHat and Debian based systems

* simplified install tasks

- reduced number of tasks in installation.packages.yml to 3
- this utilizes the package module instead of apt, yum, zypper, ...
- replaced nginx_redhat_pkg, nginx_ubuntu_pkg, nginx_freebsd_pkg, nginx_suse_pkg which all hold the same content with a new variable
- new variable nginx_pkgs contains "nginx" by default but is still a list to allow users to configure further nginx related packages they want to install

* set SELinux boolean httpd_setrlimit to allow nginx setting the rlimit

- is necessary because this role forces the setting of worker_rlimit_nofile
- introduces the need of libsemanage-python/python-semanage but since installation of SELinux modules is conditional, this does not hurt