Unified build agents

[nuclearsandwich]

I've been thinking of this as heterogeneous build agents since the agents can disparate types of jobs but the actual important part is returning to a homogeneous agent pool. So I put the Latin down and called this issue "unified" build agents.

This rollout is happening in a number of stages:

• Support heavy-job plugin in ros_buildfarm Add support for the Jenkins Heavy Job plugin ros-infrastructure/ros_buildfarm#839
• Assign a job weight of 4 on all CI jobs Add job weight of 4 to CI jobs. #140
• Re-deploy CI agents so they have 4 executors, allowing them to run heavy CI jobs
• Update Jenkins label expression and priorities for CI jobs so they can run on buildagents.
• Determine if it will be necessary to run one build-only and one ci-only agent with only the elastic agents being unified or if we can keep a fully unified agent pool.

Background: When we first put the CI jobs on the ROS build farm we ran into the issue that one CI job from ci.ros2.org which was the prototype for them is used to having a full build agent's resources rather than 1/4th of a build agent and we ran into performance issues as a result.

ros-infrastructure/ros_buildfarm#839 added support for the heavy job plugin which allows jobs to assign a "weight" which is the number of executors required to host that job.