adds dirmngr to debian install script (#3492)

* adds dirmngr to debian install script

* only install dirmngr if it is available in the cache

* better comment

Author: gmmeyer

packaging/datadog-agent/source/install_agent.sh

@@ -139,6 +139,12 @@ elif [ $OS = "Debian" ]; then
     printf "\033[34m\n* Installing apt-transport-https\n\033[0m\n"
     $sudo_cmd apt-get update || printf "\033[31m'apt-get update' failed, the script will not install the latest version of apt-transport-https.\033[0m\n"
     $sudo_cmd apt-get install -y apt-transport-https
+    # Only install dirmngr if it's available in the cache
+    # it may not be available on Ubuntu <= 14.04 but it's not required there
+    cache_output=`apt-cache search dirmngr`
+    if [ ! -z "$cache_output" ]; then
+      $sudo_cmd apt-get install -y dirmngr
+    fi
     printf "\033[34m\n* Installing APT package sources for Datadog\n\033[0m\n"
     $sudo_cmd sh -c "echo 'deb https://apt.datadoghq.com/ stable main' > /etc/apt/sources.list.d/datadog.list"
     $sudo_cmd apt-key adv --recv-keys --keyserver hkp://keyserver.ubuntu.com:80 C7A7DA52