Added new mkimage-yum.sh script to create CentOS base images

mkimage-rinse.sh requires rinse, which is not readily available on
CentOS or Fedora.  Plus, creating a base image is trivial with yum
alone.

Docker-DCO-1.1-Signed-off-by: Chris St. Pierre <chris.a.st.pierre@gmail.com> (github: stpierre)