Add Alpine Linux as supported for __filesystem.

cdist/conf/type/__filesystem/explorer/lsblk

@@ -18,16 +18,16 @@
 # along with cdist. If not, see <http://www.gnu.org/licenses/>.
 #
 
-os=$("$__explorer/os")
+os=$("${__explorer:?}/os")
 
-if [ -f "$__object/parameter/device" ]; then
+if [ -f "${__object:?}/parameter/device" ]; then
     blkdev="$(cat "$__object/parameter/device")"
 else
-    blkdev="$__object_id"
+    blkdev="${__object_id:?}"
 fi
 
 case "$os" in
-    centos|fedora|redhat|suse|gentoo)
+    alpine|centos|fedora|redhat|suse|gentoo)
         if [ ! -x "$(command -v lsblk)" ]; then
             echo "lsblk is required for __filesystem type" >&2
             exit 1