[CECO-1729] Installer support

[tbavelier]

What does this PR do?

Adds support for the Datadog installer within Puppet, including a kitchen test

Describe your test plan

Covered by kitchen E2E test and manually tested:

1. Run a Ubuntu vm as part of a Puppet cluster as agent
2. Install the installer using the following node manifest:

class { "datadog_agent":
  api_key => "REPLACE_ME",
  datadog_installer_enabled => true,
  apm_instrumentation_enabled => "host",
  apm_instrumentation_libraries => ["java:1", "python:2"],
  remote_updates => "true",
  log_level => "debug",
  agent_extra_options => {
    hostname_trust_uts_namespace => true,
  },
}

3. Ensure the installer is installed and that it added your desired packages:

/usr/bin/datadog-installer is-installed datadog-apm-library-java
# should exit code 0
/usr/bin/datadog-installer is-installed datadog-apm-library-python
# should exit code 0
/usr/bin/datadog-installer is-installed datadog-agent
# should exit code 0 if remote_updates is true, 10 otherwise

4. Verify the Agent config file is correct, includes your additional options, remote_updates with the value you set

[Pythyu]

Aside from a nit with GPG keys and a few questions LGTM overall

[Pythyu]

🚀

[BaptisteFoy]

Not a Chef expert; but overall this LGTM! It'd be nice to add DD_REMOTE_POLICIES support as well (same as DD_REMOTE_UPDATES), but let's not block this PR 😄

Thanks a lot for this