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_facts.py

@@ -59,9 +59,13 @@ options:
     default: text
     choices: ['xml', 'set', 'text', 'json']
     version_added: "2.3"
+requirements:
+  - ncclient (>=v0.5.2)
 notes:
   - Ensure I(config_format) used to retrieve configuration from device
     is supported by junos version running on device.
+  - This module requires the netconf system service be enabled on
+    the remote device being managed
 """
 
 EXAMPLES = """
@@ -79,16 +83,18 @@ ansible_facts:
   returned: always
   type: dict
 """
-
-from xml.etree.ElementTree import Element, SubElement, tostring
-
 from ansible.module_utils.basic import AnsibleModule
 from ansible.module_utils.pycompat24 import get_exception
 from ansible.module_utils.six import iteritems
 from ansible.module_utils.junos import junos_argument_spec, check_args, get_param
-from ansible.module_utils.junos import command, get_configuration
+from ansible.module_utils.junos import get_configuration
 from ansible.module_utils.netconf import send_request
 
+try:
+    from lxml.etree import Element, SubElement, tostring
+except ImportError:
+    from xml.etree.ElementTree import Element, SubElement, tostring
+
 try:
     from jnpr.junos import Device
     from jnpr.junos.exception import ConnectError