Support a custom base image

Add a new option(base-image) to support custom base image
In default, the base-image is equal to base. When using custom
image, please use command line like:

    ./tool/build.py --base-image my-image --base centos --base-tag 0.1

DocImpact

Closes-Bug: #1573544
Change-Id: If4b9c57a2b68f6f06d3e981f30b2173064e02193