Instruct NetPlan to use systemd-networkd

systemd-networkd is used by default with Ubuntu Server. Installing ubuntu-desktop (as part of DCV installation) installs NetworkManager. NetworkManager is more complex (with WiFi capabilities) and causes confusion to systemd-networkd. When systemd-networkd is confused, it delays the boot by 2 minutes.

This commit instructs NetPlan to use systemd-networkd to manage network interfaces. The code is added at the end of DCV installation because the mitigation is strictly related to the installation of ubuntu-desktop. Always using systemd-networkd also improves consistency between how ParallelCluster handles single-nic instances vs multi-nic instances. With multi-nic instances ParallelCluster has been instructing netplan to use systemd-networkd ([code](https://github.com/aws/aws-parallelcluster-cookbook/blob/develop/cookbooks/aws-parallelcluster-environment/files/ubuntu/network_interfaces/configure_nw_interface.sh#L62))

# Technical details:
## Output of `networkctl list`
### Prior to this commit
Base Ubuntu:
```
IDX LINK TYPE     OPERATIONAL SETUP
  1 lo   loopback carrier     unmanaged
  2 ens5 ether    routable    configured

2 links listed.
```
Ubuntu with ubuntu-desktop
```
IDX LINK TYPE     OPERATIONAL SETUP
  1 lo   loopback carrier     unmanaged
  2 ens5 ether    routable    unmanaged

2 links listed.
```
systemd-networkd got confused because it saw no network interface was setup (because NetworkManager took over control of all network interfaces) and waited until 2 minutes timeout at the beginning of system boot:
```
$ journalctl -b | grep -i "ipv6\|timeout\|waiting"
...
Sep 18 14:51:23 systemd-networkd-wait-online[1602]: Timeout occurred while waiting for network connectivity.
...
Sep 18 14:53:31 systemd-networkd-wait-online[1891]: Timeout occurred while waiting for network connectivity.
...
```
### After this commit
Ubuntu with ubuntu-desktop has the same output as Base Ubuntu and the delay is gone
Signed-off-by: Hanwen <hanwenli@amazon.com>

Author: hanwen-cluster

cookbooks/aws-parallelcluster-platform/resources/dcv/partial/_ubuntu_common.rb

@@ -81,4 +81,20 @@ def optionally_disable_rnd
       command "sed --in-place '/RANDFILE/d' /etc/ssl/openssl.cnf"
     end
   end
+
+  def post_install
+    # ubuntu-desktop comes with NetworkManager. On a cloud instance NetworkManager is unnecessary and causes delay.
+    # Instruct Netplan to use networkd for better performance
+    bash 'Instruct Netplan to use networkd' do
+      code <<-NETPLAN
+        set -e
+        cat > /etc/netplan/95-parallelcluster-force-networkd.yaml << 'EOF'
+network:
+  version: 2
+  renderer: networkd
+EOF
+        netplan apply
+      NETPLAN
+    end unless on_docker?
+  end
 end