add option to install extra packages on agents

defaults/main.yml

@@ -50,6 +50,11 @@ droneci_autoscaler_agent_architecture: amd64
 droneci_autoscaler_agent_volumes: []
 droneci_autoscaler_agent_package_upgrade: False
 
+# @var droneci_autoscaler_agent_packages:description: >
+# Install extra required packages during agents bootstrapping.
+# @end
+droneci_autoscaler_agent_packages: []
+
 # @var droneci_autoscaler_docker_auth_config:description: Docker registries auth config file (config.json).
 # @var droneci_autoscaler_docker_auth_config: >
 # droneci_autoscaler_docker_auth_config: |

templates/services/userdata.yml.j2

@@ -17,6 +17,9 @@ apt:
       keyid: 9DC858229FC7DD38854AE2D88D81803C0EBFCD88
 
 packages:
+  {% for package in inst.agent_packages | default(droneci_autoscaler_agent_packages) %}
+  - {{ package }}
+  {% endfor %}
   - bridge-utils
   - 'docker-ce{{ ':armhf' if inst.agent_architecture | default(droneci_autoscaler_agent_architecture) == "arm" else '' }}'