For some use cases, it can be useful to keep all the kernels
and not just keep the latest one. Add a parameter that allows
it, and continue cleaning up kernels by default.
Change-Id: Ia6e6c1fa18e3724c1eb89226151d81e9e748b793
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
