Print agent specification upon node allocation

[Vlatombe]

First shot at printing agent specification upon first use.

This prints the docker image used for each container as well as the resource constraints if they are defined.

[carlossg]

This is the output

Agent jenkins-slave-3q8tf-w9dk6 is provisioned from template Kubernetes Pod Template
Agent specification [Kubernetes Pod Template] (mypod):
* [jnlp] jenkins/jnlp-slave:3.10-1-alpine
* [maven] maven:3.3.9-jdk-8-alpine
* [golang] golang:1.6.3-alpine

Running on jenkins-slave-3q8tf-w9dk6 in /home/jenkins/workspace/p