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

lib/ansible/modules/network/junos/junos_rpc.py

@@ -55,6 +55,11 @@ options:
         version of software that supports native JSON output.
     required: false
     default: xml
+requirements:
+  - ncclient (>=v0.5.2)
+notes:
+  - This module requires the netconf system service be enabled on
+    the remote device being managed
 """
 
 EXAMPLES = """
@@ -84,8 +89,6 @@ output_lines:
   returned: always
   type: list
 """
-from xml.etree.ElementTree import Element, SubElement, tostring
-
 from ansible.module_utils.junos import junos_argument_spec, check_args
 from ansible.module_utils.basic import AnsibleModule
 from ansible.module_utils.netconf import send_request
@@ -93,6 +96,11 @@ from ansible.module_utils.six import iteritems
 
 USE_PERSISTENT_CONNECTION = True
 
+try:
+    from lxml.etree import Element, SubElement, tostring
+except ImportError:
+    from xml.etree.ElementTree import Element, SubElement, tostring
+
 
 def main():
     """main entry point for Ansible module