Rename cloud-infra-verify jjb file and the jobs

This change renames the existing cloud-infra-verify-jobs.yaml file
as cloud-infra-verify-engine.yaml in order to prepare for creating
jobs to verify changes sent to cloud-infra/test repository. The jobs
that will test changes sent to cloud-infra/test will be declared in
cloud-infra-verify-test.yaml.

The jobs that test changes coming to cloud-infra/engine repo are
also renamed from cloud-infra-verify-{distro}-virtual-master to
cloud-infra-verify-engine-{distro}-virtual-master.

Additional jobs will be created for verifying changes coming to
infra/swconfig and infra/hwconfig once we start utilizing these repos.

Apart from that, a comment to explain what the jobs declared in
cloud-infra-verify-deploy.yaml do, what are the prerequisies, and
so on are included.

Finally, support for ubuntu1604 is dropped as we moved to ubuntu18.04
and switched to coreos for ipa image.

Change-Id: I109637d51257fba7ccbae96128814e2974979089