[AGENTONB-2490] Support remote_updates through embedded installer

[tbavelier]

What does this PR do?

• Stops managing Agent through deprecated installer and moves installer steps to the end (ensure it is done after Agent installation if there's one to avoid doing 2 keys installation on Ubuntu) c21e93c
• Adds a new fact to check if an upgrade is running (datadog-agent-exp service), and if yes, moves ensure running (i.e. Puppet restarts the Agent service) to stopped to avoid interfering with the remote upgrade
  • Once the upgrade is over, at the next Puppet runs, we get back to the original state with running (as the fact will not be true): da205b3

Motivation

• Same as [AGENTONB-2490] Support remote_updates through embedded installer ansible-datadog#665 but for Puppet

Additional Notes

Describe your test plan

• Unit/kitchen tests to ensure default behaviour is not changed
• Manually tested with VMs the following flow:
  1. Agent is installed with remote_updates, service is started
  2. Upgrade is started, Agent is not re-started despite service being dead
  3. Did a config change (datadog.yaml), and ensured Puppet did not restart Agent service
  4. Upgrade is finished, manually stopped Agent service and ensured Puppet did restart it (ensure => running)