Allow changing base image

By changing the PREFIX variable in the .buildconf one is now able to
build docker images from different bases.

For example, add the following line to your .buildconf file to build
CentOS based images:

    PREFIX=centos-rdo-

Default base image is Fedora. For now only RH family is supported.

Additionally, changing the namespace either with the NAMESPACE variable
in .buildconf or via --namespace commandline option now changes the
source namespace as well from the default kollaglue one.

Implements: blueprint multi-baseos
Co-Authored-By: Steven Dake <stdake@cisco.com>
Change-Id: I3964cd2292789ea883a1f2d2738a5731a4fff49b