Remove `external_url` from ini so that cookbook work with jenkins cookbook > v2.0.0

## Problem
This cookbook causes the chef run to fail if using jenkins cookbook >= 2.0.0.

## Context
jenkins-job-builder's ini no longer requires `external_url` (replaced by `url`?):
http://ci.openstack.org/jjb.html#sending-a-job-to-jenkins

Currently, this cookbook depends on a `node['jenkins']['server']['url']` being set:
https://github.com/needle-cookbooks/chef-jenkins-job-builder/blob/master/templates/default/jenkins_jobs.ini.erb#L5

However, this is no longer an attribute in the redesigned jenkins cookbook, and the closest thing is [`node['jenkins']['master']['host']`](https://github.com/needle-cookbooks/chef-jenkins-job-builder/blob/master/templates/default/jenkins_jobs.ini.erb#L5) (which still might not be right to hardcode).

## Solution
Can we simply remove that line so that this cookbook will work with the new jenkins cookbook?