(MODULES-6092) Set correct seluser for CentOS/RHEL 5.x

(MODULES-6092) Set correct seluser for CentOS/RHEL 5.x

In 1.10.0, applying the puppetlabs-firewall module on CentOS/RHEL 5.x produces warnings:

Warning: Failed to set SELinux context unconfined_u:object_r:etc_t on /etc/sysconfig/iptables
Notice: /Stage[main]/Firewall::Linux::Redhat/File[/etc/sysconfig/iptables]/seluser: seluser changed 'system_u' to 'unconfined_u'
Warning: Failed to set SELinux context unconfined_u:object_r:etc_t on /etc/sysconfig/ip6tables
Notice: /Stage[main]/Firewall::Linux::Redhat/File[/etc/sysconfig/ip6tables]/seluser: seluser changed 'system_u' to 'unconfined_u'

This commit adds a case to set the correct seluser.