[BuildImage] Prevent service restarts during installation of ubuntu-desktop, required by DCV, to avoid SSM agent connecgtivity issue.

In particular:
* Add NEEDRESTART_MODE=l environment variable to suppress needrestart prompts during installation

Author: gmarciani

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

@@ -45,6 +45,7 @@ def pre_install
       code <<-PREREQ
         set -e
         DEBIAN_FRONTEND=noninteractive
+        NEEDRESTART_MODE=l
         apt -y install whoopsie
         apt -y install ubuntu-desktop && apt -y install mesa-utils || (dpkg --configure -a && exit 1)
         apt -y purge ifupdown

cookbooks/aws-parallelcluster-platform/spec/unit/resources/dcv_spec.rb

@@ -472,6 +472,7 @@ def self.nothing(chef_run)
           when 'ubuntu'
             is_expected.to periodic_apt_update('')
             is_expected.to run_bash('install pre-req').with_cwd(Chef::Config[:file_cache_path]).with_retries(10).with_retry_delay(5)
+                                                      .with_code(/NEEDRESTART_MODE=l/)
                                                       .with_code(/apt -y install whoopsie/)
                                                       .with_code(/apt -y install ubuntu-desktop && apt -y install mesa-utils || (dpkg --configure -a && exit 1)/)
                                                       .with_code(/apt -y purge ifupdown/)