Add xconnect dcnm network

[ccoueffe]

fix: #604

Add xconnect for ND 4.1:

    - cisco.dcnm.dcnm_network:
        state: merged
        fabric: myFab
        config:
          - net_name: TEST-XCONNECT-NET1
            is_l2only: true
            vlan_id: 2001
            xconnect: true

changed: [myFab] => {
    "changed": true,
    "diff": [
        {
            "arp_suppress": false,
            "attach": [],
            "dhcp_loopback_id": "",
            "dhcp_servers": "",
            "dhcp_srvr1_ip": "",
            "dhcp_srvr1_vrf": "",
            "dhcp_srvr2_ip": "",
            "dhcp_srvr2_vrf": "",
            "dhcp_srvr3_ip": "",
            "dhcp_srvr3_vrf": "",
            "gw_ip_subnet": "",
            "gw_ipv6_subnet": "",
            "int_desc": "",
            "intfvlan_nf_monitor": "",
            "is_l2only": true,
            "l3gw_on_border": false,
            "mtu_l3intf": "",
            "multicast_group_address": "239.1.1.1",
            "net_extension_template": "Default_Network_Extension_Universal",
            "net_id": 30000,
            "net_name": "TEST-XCONNECT-NET1",
            "net_template": "Default_Network_Universal",
            "netflow_enable": false,
            "route_target_both": false,
            "secondary_ip_gw1": "",
            "secondary_ip_gw2": "",
            "secondary_ip_gw3": "",
            "secondary_ip_gw4": "",
            "trm_enable": false,
            "vlan_id": 2001,
            "vlan_name": "",
            "vlan_nf_monitor": "",
            "vrf_name": "NA",
            "xconnect": true
        }
    ],
    "invocation": {
        "module_args": {
            "config": [
                {
                    "is_l2only": true,
                    "net_name": "TEST-XCONNECT-NET1",
                    "vlan_id": 2001,
                    "xconnect": true
                }
            ],
            "fabric": "myFab",
            "state": "merged"
        }
    },
    "response": [
        {
            "DATA": {},
            "MESSAGE": "OK",
            "METHOD": "PUT",
            "REQUEST_PATH": "https://x.x.x.x:443/appcenter/cisco/ndfc/api/v1/lan-fabric/rest/top-down/fabrics/myFab/networks/TEST-XCONNECT-NET1",
            "RETURN_CODE": 200
        }
    ]
}

PLAY RECAP *********************************************************************************************************************************************************************************************************************
myFab                 : ok=1    changed=1    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0   

[allenrobel]

This looks fine to me. The only minor points would be:

1. We might want to add integration test for xconnect.
2. Since there is no 4.x version check around xconnect, what happens when xconnect is specified in a playbook task and run against ND 3.x?

I know adding unit tests would be difficult due to the way UT are structured for the dcnm_network module, so I'm fine with not adding to or modifying unit tests.

I will leave it to @mikewiebe to approve.

Thank you!