merge_vars: fix shared-prefix conflicts

Example: the suffix "files__to_merge" also matches
"wireless_profiles__to_merge" which caused funny issues.

Let's also match on the leading underscores to prevent that.

roles/cfg_openwrt/tasks/merge_vars.yml

@@ -1,50 +1,50 @@
 ---
 - name: Merge packages variable
   merge_vars:
-    suffix_to_merge: packages__to_merge
+    suffix_to_merge: __packages__to_merge
     merged_var_name: packages
     expected_type: list
 
 - name: Merge disabled_services variable
   merge_vars:
-    suffix_to_merge: disabled_services__to_merge
+    suffix_to_merge: __disabled_services__to_merge
     merged_var_name: disabled_services
     expected_type: list
 
 - name: Merge wireless_profiles variable
   merge_vars:
-    suffix_to_merge: wireless_profiles__to_merge
+    suffix_to_merge: __wireless_profiles__to_merge
     merged_var_name: wireless_profiles
     expected_type: list
 
 - name: Merge ssh_keys variable
   merge_vars:
-    suffix_to_merge: ssh_keys__to_merge
+    suffix_to_merge: __ssh_keys__to_merge
     merged_var_name: ssh_keys
     expected_type: list
   when: ssh_keys is undefined
 
 - name: Merge sysctl variable
   merge_vars:
-    suffix_to_merge: sysctl__to_merge
+    suffix_to_merge: __sysctl__to_merge
     merged_var_name: sysctl
     expected_type: dict
 
 - name: Merge rclocal variable
   merge_vars:
-    suffix_to_merge: rclocal__to_merge
+    suffix_to_merge: __rclocal__to_merge
     merged_var_name: rclocal
     expected_type: list
 
 - name: Merge files variable
   merge_vars:
-    suffix_to_merge: _files__to_merge
+    suffix_to_merge: __files__to_merge
     merged_var_name: files
     expected_type: list
 
 - name: "Merge channel_assignments variables"
   merge_vars:
-    suffix_to_merge: "{{ item.split('__')[1] + '__to_merge' }}"
+    suffix_to_merge: "__{{ item.split('__')[1] + '__to_merge' }}"
     merged_var_name: "{{ item.split('__')[1] }}"
     expected_type: dict
   loop: "{{ hostvars[inventory_hostname] | select('match', '.*__channel_assignments_.*') }}"