datadog-signing-keys install: fix order of execution (#721)

Fix error "Unable to locate package datadog-signing-keys" on machines that
still didn't have the datadog repo because "apt install" was triggered
before "apt update". This doesn't make "apt update" trigger twice.

CHANGELOG.md

@@ -4,7 +4,8 @@ Changes
 # 3.14.0 / 2021-09-29
 
 * [FEATURE] Support Raspbian as debian-based systems ([#719][]) (Thanks [@Mstrodl])
-* [BUGFIX] Add support for security-agent config ([#706][]) (Thanks [@florusboth])
+* [FEATURE] Add support for security-agent config ([#706][]) (Thanks [@florusboth])
+* [BUGFIX] Fix "Unable to locate package datadog-signing-keys" error on new installs ([#721][])
 
 # 3.13.0 / 2021-08-11
 
@@ -864,6 +865,7 @@ Please read the [docs]() for more details.
 [#712]: https://github.com/DataDog/puppet-datadog-agent/issues/712
 [#714]: https://github.com/DataDog/puppet-datadog-agent/issues/714
 [#719]: https://github.com/DataDog/puppet-datadog-agent/issues/719
+[#721]: https://github.com/DataDog/puppet-datadog-agent/issues/721
 [@Aramack]: https://github.com/Aramack
 [@BIAndrews]: https://github.com/BIAndrews
 [@ChannoneArif-nbcuni]: https://github.com/ChannoneArif-nbcuni
@@ -979,4 +981,4 @@ Please read the [docs]() for more details.
 [@yrcjaya]: https://github.com/yrcjaya
 [@zabacad]: https://github.com/zabacad
 [@zickzackv]: https://github.com/zickzackv
-[@zoom-kris-anderson]: https://github.com/zoom-kris-anderson
+[@zoom-kris-anderson]: https://github.com/zoom-kris-anderson

manifests/ubuntu.pp

@@ -107,6 +107,8 @@
   }
 
   package { 'datadog-signing-keys':
-    ensure => 'latest',
+    ensure  => 'latest',
+    require => [Apt::Source['datadog'],
+                Class['apt::update']],
   }
 }