The centos cloud images are both arround a GB in size, and
downloading them causes lot of CI timeouts, downloading the
compressed version saves 700MB of bandwith and should save
a lot of time.
Change-Id: I8dcd1db81fe5c4661945638ef3e6344fdf651243
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
