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.
Perhaps better to say "Standalone CDAP" - because this is for sure not a good setting for distributed CDAP.