Create drop directories for plugins and init scripts

So that the image can be easily customized downstream

The problem with build time customization is that plugins can't be copied into JENKIS_HOME, being a volume.
Here a drop dir is created at /usr/share/jenkins/plugins (same for groovy init scripts) and files are copied at runtime into JENKINS_HOME
So if you want to customize the image just copy plugins there and they will be automatically installed:

```
FROM jenkins
RUN curl -sSL -o /usr/share/jenkins/plugins/swarm.hpi https://updates.jenkins-ci.org/latest/swarm.hpi
```

Some of that is done in #19 too

WDYT?