This is the centos element, which builds CentOS 6.
There are a couple of modifications to redhat-common because
the version of tar in el6 doesn't support --xattrs-include.
There is a change to both pkg-map and svc-map to add 'centos'
to the 'redhat' family.
Also explicitly have to install cloud-utils growroot and
dracut growpart for proper fsresize at instance launch.
Also sets the DIB_EXTLINUX variable because there is no grub2
for this distro.
Change-Id: Iffd57bce1484c43c2cffcbdb37b602185216e63a
