TThis PR fixes the default value of docker images and adds docuemtation
fro building docker container images.
The default values of docker image should be something that makes users
change the value before they start using Trove because Trove team
currently does not plan to maintain the following docker images on
https://hub.docker.com/u/openstacktrove
Since Victoria, Trove switched GuestAgent's image from VM to Docker,
however the building guest image docs does not contain how to build
docker container images.
Co-Authored-By: wu.chunyang <wchy1001@gmail.com>
Story: 2010804
Task: 48288
Change-Id: I7047d888c4340f16ca2a1a77b5f9c34ba75b5a30
67e1389221