Updates to installation instructions for Virtual Machine and Docker images

Staged at http://stg-web101.sw.joyent.continuuity.net/cdap/2.5.0/en/getstarted.html
Listed and created sections for the three different download methods.
Updated instructions for Virtual Machine and Docker.
Port 9999 is not the only port that needs to be published, we also need to publish port 10000 for the REST API to work. I tried that and there seems to be an issue with docker, it refuses o publish port 10000. @Dawoudibrahim is investigating how to fix that, I am also trying to figure it out, we need to wait for resolution.