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.
I just noticed that further down you call this the Host Interface IP Adress. You should name it the same in all places. I believe that Docker uses Host Interface IP Address, that is probably why it is here. I feel that Docker VM's IP address is more descriptive. Your call.