Adds SELinux custom module for the afterburn systemd units

the afterburn systemd units fail as the SELinux domain of the afterburn binary is restricted from changing the content of files in /run, /run/metadata and /home/$user/.ssh. This commit adds a afterburn-custom.cil SELinux module to allow the afterburn services to succeed and the nodes to properly join a cluster. The module is loaded by the okd-selinux.service implemented by 336013f

Refers #1555

overlay.d/50scos/usr/lib/okd/selinux/afterburn.cil

@@ -0,0 +1,7 @@
+(typeattributeset cil_gen_require var_run_t)
+(typeattributeset cil_gen_require afterburn_t)
+(typeattributeset cil_gen_require ssh_home_t)
+(allow afterburn_t ssh_home_t (dir (read search)))
+(allow afterburn_t ssh_home_t (dir (open)))
+(allow afterburn_t var_run_t (dir (add_name create write)))
+(allow afterburn_t var_run_t (file (create open write)))