[Enhancement] Adding an entrypoint to allow custom Global Tools modifications

[peterzhuamazon]

Right now we are directly adding custom global tools configurations here:

• Add JENKINS_HOME_PATH as default JENKINS_HOME always returns /var/jenkins_home and update jdk14 #215

This is not generic enough and we should follow the envVars approach here, by adding a class to read from a custom configurations files if we need any custom tweaks on the global tools:

• Add environment variables using configuration as code #136

Thanks.