explorer/memory: Support OpenBSD (#602)

Adds support to detect the amount of memory available on OpenBSD
systems.

cdist/conf/explorer/memory

@@ -2,6 +2,7 @@
 #
 # 2014 Daniel Heule  (hda at sfs.biz)
 # 2014 Thomas Oettli (otho at sfs.biz)
+# Copyright 2017, Philippe Gregoire <pg@pgregoire.xyz>
 #
 # This file is part of cdist.
 #
@@ -28,6 +29,10 @@ case "$os" in
         echo "$(sysctl -n hw.memsize)/1024" | bc
     ;;
 
+    "openbsd")
+        echo "$(sysctl -n hw.physmem) / 1048576" | bc
+    ;;
+
     *)
     if [ -r /proc/meminfo ]; then
         grep "MemTotal:" /proc/meminfo | awk '{print $2}'