Add support for Bearer Token Authentication

[bsquizz]

When Jenkins is running on top of OpenShift, bearer token authentication is used for all REST API calls.

This means the API requests are authenticated via a header like:
Authorization: Bearer <Kubernetes/OpenShift Token>

I've been using these modifications to the plugin to trigger remote jobs on a master deployed on OpenShift with the OpenShift authentication enabled.

Example usage in a pipeline:

    def jobParameters = [[$class: StringParameterValue, name: "someParam", value: "someValue"]]
    // Translate the params into a string
    def paramsString = ""
    for (Map p : jobParameters) {
        paramsString = paramsString + "\n${p['name']}=${p['value'].toString()}"
    }

    withCredentials([
        string(credentialsId: remoteCredentials, variable: "TOKEN"),
        string(credentialsId: remoteHostname, variable: "REMOTE_HOSTNAME")
    ]) {
        triggerRemoteJob(
            job: "https://${REMOTE_HOSTNAME}/job/path/to/job",
            parameters: paramsString,
            auth: BearerTokenAuth(token: TOKEN)
        )
    }

[cashlalala]

looks good, thanks for the contribution

[daniel-beck]

This has no UI in the global configuration, and the HTML help hasn't been updated either (in a while, it seems).