Add junos_system declartive module and other related change (#25859)

* Add junos_system declartive module and other related change

*  junos_system declartive module
*  integration test for junos_system
*  integration test for net_system (junos platform)
*  pep8 fixes for junos modules
*  move to lxml from elementree for xml parsing as it support
   complete set of xpath api's
*  other minor changes

* Fix CI and doc changes

* Fix unit test failures

* Fix typo in import

* Fix import issue for py2.6

* Add missed Element in import

test/integration/targets/junos_system/tasks/main.yaml

@@ -0,0 +1,2 @@
+---
+- { include: netconf.yaml, tags: ['netconf'] }

test/integration/targets/junos_system/tasks/netconf.yaml

@@ -0,0 +1,16 @@
+---
+- name: collect all netconf test cases
+  find:
+    paths: "{{ role_path }}/tests/netconf"
+    patterns: "{{ testcase }}.yaml"
+  register: test_cases
+  delegate_to: localhost
+
+- name: set test_items
+  set_fact: test_items="{{ test_cases.files | map(attribute='path') | list }}"
+
+- name: run test case
+  include: "{{ test_case_to_run }}"
+  with_items: "{{ test_items }}"
+  loop_control:
+    loop_var: test_case_to_run