Fix junos modules check_mode issue (#37311)

Fixes #37208

If check_mode is enabled instead of committing th config need to
discard all the chnages to cnadidate db
In case of cli to discard changes issue `rollback 0` command
and for netconf execute `discard-changes` rpc call

test/integration/targets/junos_banner/tests/netconf/basic.yaml

@@ -78,6 +78,20 @@
       - "result.changed == true"
       - "'<message>this is my login banner</message>' in config.xml"
 
+- name: check mode
+  junos_banner:
+    banner: login
+    text: this is not the login banner you're looking for
+    state: present
+    provider: "{{ netconf }}"
+  register: result
+  check_mode: yes
+
+- assert:
+    that:
+      - "result.changed == true"
+      - "result.failed == false"
+
 - name: delete login banner
   junos_banner:
     banner: login