[stable/jenkins] Create agent-jcasc template to configure multiple agents

[wmcdona89]

What this PR does / why we need it:

This PR provides the ability to configure multiple agents via JCasC. See the README for an example.

Which issue this PR fixes

(optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged)

• fixes [stable/jenkins] Support Multiple Kubernetes podTemplates helm/charts#20870

Special notes for your reviewer:

The pod template definition (kubernetes.templates) in template jenkins.casc.defaults is moved to a new jenkins.agent-jcasc template with the following modifications. See http://www.mergely.com/h9N9IvrT/?ws=1.

1. name fields moved to the top so they stand out
2. command quoted when not nil to support empty string (empty string ensures Jenkins doesn't apply a default value such as /bin/sh -c)
3. empty checks added for resources, resources.limits, resources.requests, and yamlTemplate
4. workingDir parameterized
5. volumes added

I chose not to make use of the kubernetes plugin casc fields related to agent inheritance.

defaultsProviderTemplate - the agent from which all other agents inherit configuration
Merging agent into the other agents in the JCasC allows all agents to make use of the chart's default configuration for agent. To wait for the kubernetes plugin to merge the values at runtime means each agent would need to set sideContainerName (so container values could be inherited) as well as set any fields which get assigned a default value when nil and thus aren't eligible for inheritance (e.g. command and workingDir).

inheritFrom - indicates from which agent this agent should inherit configuration
I thought an agent.inheritFrom field would look out of place in the chart since the chart only defines a single agent.

helm template tests

for helm 2 use --execute instead of --show-only

auto-reload

default JCasC config

helm template stable/jenkins \
--show-only templates/jcasc-config.yaml \
--set master.sidecars.configAutoReload.enabled=true \
--set master.JCasC.enabled=true \
--set master.JCasC.defaultConfig=true

JCasC ConfigScripts

stable/jenkins/agent-values.yaml contains the example from the README

helm template stable/jenkins \
--show-only templates/jcasc-config.yaml \
--set master.sidecars.configAutoReload.enabled=true \
--set master.JCasC.enabled=true \
--set master.JCasC.defaultConfig=false \
--values stable/jenkins/agent-values.yaml

NO auto-reload

default JCasC config

helm template stable/jenkins \
--show-only templates/config.yaml \
--set master.sidecars.configAutoReload.enabled=false \
--set master.JCasC.enabled=true \
--set master.JCasC.defaultConfig=true

JCasC ConfigScripts

stable/jenkins/agent-values.yaml contains the example from the README

helm template stable/jenkins \
--show-only templates/config.yaml \
--set master.sidecars.configAutoReload.enabled=false \
--set master.JCasC.enabled=true \
--set master.JCasC.defaultConfig=false \
--values stable/jenkins/agent-values.yaml

Checklist

[Place an '[x]' (no spaces) in all applicable fields. Please remove unrelated fields.]

• DCO signed
• Chart Version bumped
• Variables are documented in the README.md
• Title of the PR starts with chart name (e.g. [stable/mychartname])