TT#72908 Change compat symlink check from /var/backup/cdr to /var/backup

Let us introduce a system-wide symlink from /var/backup to /ngcp-data/backup, to
fix recurrent problems like the missing /var/backup/cdr or
/var/backup/ngcp-upgrade backups taking too much space.

This should have happened when changing the partition schema to use /ngcp-data/
and have a root partition with minimal amount of space, because there were
several paths migrated to /ngcp-data like /var/backup, and we failed to migrate
all those paths in sync with the change to the partitioning and also failed to
have a back-up solution like this symlink.

Some (hopefully most, or all!) of the problems have been sorted out by now, but
they've been biting us years after the change, like this TT#72908, so maybe it
can still save some pain in the future, and there's little cost to it.

Change-Id: Ief4ba33bf57bb5f02cc54b000b64540667ebba48
(cherry picked from commit 25d4f5e9c1,
fixing whitespace)

templates/130_filesys-ngcp.yaml.tt2

@@ -236,10 +236,10 @@ file:
     group: root
     filetype: directory
 
-  /var/backup/cdr:
+  /var/backup:
     exists: true
     mode: "0777"
     owner: root
     group: root
-    linked-to: /ngcp-data/backup/cdr
+    linked-to: /ngcp-data/backup
     filetype: symlink