Trust new APT and RPM keys

[mx-psi]

What does this PR do?

• Trust new APT and RPM keys
• Amend spec tests

Describe your test plan

• Manual tests and also run CI test suite locally with bundle exec rake test

Manual tests

Apply the module with the manifests below, it should install the Agent if using this branch's module and fail if using master.
Apply a module with no options, it should install correctly.

Ubuntu

class { 'datadog_agent':
    api_key => "notnullapikey",
    agent_repo_uri => '<test repo goes here>',
    apt_release => 'nightly',
    agent_version => '7.23.0~devel.git.35.4b4d662',
}

Redhat

class { 'datadog_agent':
    api_key => "notnullapikey",
    agent_repo_uri => '<test repo goes here>/nightly/7/x86_64/',
    agent_version => '7.23.0~devel.git.37.5884362',
}

[mx-psi]

Mind taking a second look @KSerrania?