Simplify image building information in quickstart

Remove information about build time on specific hardware and reasons
for not pushing images per commit that are not relevant for
a quickstart guide.

Link to detailed instruction about image building from the quickstart
guide and update the image building guide to use `kolla-build` command
by default instead of `tools/build.py`.

Change-Id: If821370acdaaa860c96597aa373687f5f10d3121
Partially-implements: blueprint documentation-rework