add selinux category range to config

This allows an admin to set the upper bounds on the category range for selinux
labels.  This can be useful when handling allocation of PVs or other volume
types that need to be shared with selinux enabled on the hosts and volumes.

Signed-off-by: Michael Crosby <michael@thepasture.io>

docs/config.md

@@ -35,6 +35,10 @@ version = 2
   # enable_selinux indicates to enable the selinux support.
   enable_selinux = false
 
+  # selinux_category_range allows the upper bound on the category range to be set.
+  # if not specified or set to 0, defaults to 1024 from the selinux package.
+  selinux_category_range = 1024
+
   # sandbox_image is the image used by sandbox container.
   sandbox_image = "k8s.gcr.io/pause:3.2"