[CECO-1804] Add remote policies

[tbavelier]

What does this PR do?

1. Support for DD_REMOTE_POLICIES when using the installer, same behaviour as DD_REMOTE_UPDATES: 54b1765
2. Link the datadog-agent service to the installer when the Agent is managed by it: 2e14d9b

Motivation

1. Request from fleet automation team
2. Using the installer with DD_REMOTE_UPDATES=true, the Agent package is installed with it but the service was not started as is for a normal Agent install as in [CECO-1729] Installer support #820, I gated the service behind non-installer managed Agent installations. This commit fixes that by defining the Agent service with the datadog-installer flavor and reverting the previous gate (return to original code for the management of /etc/datadog-agent/datadog.yaml before [CECO-1729] Installer support #820 ):

   puppet-datadog-agent/manifests/init.pp

   Lines 782 to 790 in 9184de0

   	file { '/etc/datadog-agent/datadog.yaml':
   	owner => $dd_user,
   	group => $dd_group,
   	mode => '0640',
   	content => template('datadog_agent/datadog.yaml.erb'),
   	show_diff => false,
   	notify => Service[$datadog_agent::params::service_name],
   	require => File['/etc/datadog-agent'],
   	}

Describe your test plan

1. Use the following site.pp:

   class { "datadog_agent":
     api_key => "REPLACE_ME",
     datadog_site => "datad0g.com",
     manage_install => false,
     datadog_installer_enabled => true,
     remote_updates => true,
     remote_policies => true,
   }

2. Ensure the DD Agent service is running after applying your manifest: systemctl status datadog-agent
3. Verify remote_policies: true is in datadog.yaml
4. Add a configuration option modifying datadog.yaml in your site.pp, e.g.:

   class { "datadog_agent":
     api_key => "REPLACE_ME",
     datadog_site => "datad0g.com",
     manage_install => false,
     datadog_installer_enabled => true,
     remote_updates => true,
     remote_policies => true,
     agent_extra_options => {
       foo => "bar",
     },
   }

5. Ensure the Agent service was restarted