[explorer/os_version] Convert Devuan ceres to version number

Conversion of Devuan ceres to version numbers is done based on Devuan codenames.
The version number is the version number of the final release - 0.01.

Analogous to Debian.

cdist/conf/explorer/os_version

@@ -1,6 +1,7 @@
-#!/bin/sh
+#!/bin/sh -e
 #
 # 2010-2011 Nico Schottelius (nico-cdist at schottelius.org)
+# 2020-2021 Dennis Camera (dennis.camera at ssrq-sds-fds.ch)
 #
 # This file is part of cdist.
 #
@@ -17,12 +18,11 @@
 # You should have received a copy of the GNU General Public License
 # along with cdist. If not, see <http://www.gnu.org/licenses/>.
 #
-#
 # All os variables are lower case
 #
-#
 
-case "$("$__explorer/os")" in
+case $("${__explorer:?}/os")
+in
    amazon)
       cat /etc/system-release
    ;;
@@ -59,7 +59,23 @@ case "$("$__explorer/os")" in
       esac
    ;;
    devuan)
-      cat /etc/devuan_version
+      devuan_version=$(cat /etc/devuan_version)
+      case ${devuan_version}
+      in
+         (*/ceres)
+            # ceres versions don't have a number, so we decode by codename:
+            case ${devuan_version}
+            in
+               (chimaera/ceres) echo 3.99 ;;
+               (beowulf/ceres) echo 2.99 ;;
+               (ascii/ceres) echo 1.99 ;;
+               (*) exit 1
+            esac
+            ;;
+         (*)
+            echo "${devuan_version}"
+            ;;
+      esac
    ;;
    fedora)
       cat /etc/fedora-release