Fix overriding HOME env var and remove duplicated env vars

[kayano]

This PR enables redefining default env variables like HOME and removes duplicated env vars for created containers.
It fixes regression introduced in https://github.com/jenkinsci/kubernetes-plugin/pull/162/files#diff-14316beac1f1489a56a52e21cf8e0ee6R371

After bumping kubernetes-plugin to version 1.0 redefining HOME env variable doesn't work.
One of our pipeline where we need to redefine HOME variable for a container in a podTemplate stopped working.

podTemplate(label: 'mypod',
    containers: [
        containerTemplate(
            name: 'mongodb', 
            image: 'bitnami/mongodb:3.4.5-r0',
            envVars: [
                envVar(key: 'HOME', value: '/root'),
            ]
        )
    ]
)
{
    node ('mypod') {
        stage('Check mongo') {
            container('mongodb') {
                sh """
                mongo version
                """
            }
        }
    }
}

On kubernetes-plugin version 1.0 mongodb container starts with following setup:

Environment variables:
HOME: /root
JENKINS_SECRET: <secret>
JENKINS_NAME: jenkins-slave-tg7rl-mcknd
JENKINS_URL: http://192.168.99.1:8080/jenkins/
HOME: /home/jenkins

And the pod terminates with an error from mongodb container:

INFO  ==> Starting mongod...
error: failed switching to "mongo": unable to find user mongo: no matching entries in passwd file

Environment variables should look like here for mongodb container from the example:

HOME: /root
JENKINS_SECRET: <secret>
JENKINS_NAME: jenkins-slave-tg7rl-mcknd
JENKINS_URL: http://192.168.99.1:8080/jenkins/

[marvinthepa]

If you use single quotes, you don't have to escape the $..

[marvinthepa]

dito

[marvinthepa]

This means "first one wins", correct?

Is this really what we want?

[marvinthepa]

For consistency (and because it is shorter), isn't this the same as:

        env.entrySet().forEach(item ->
                envVarsMap.computeIfAbsent(item.getKey(), k -> new EnvVar(item.getKey(), item.getValue(), null))
        );

[kayano]

@marvinthepa thank you for your review. Is it ok now?

[marvinthepa]

Looks good to me now.

I remember @carlossg saying that KubernetesPipelineTest is already to large, so maybe that test could be split out (maybe with others concerning environment variable handling).