Remove cloud-init from xenial repository

We built custom version of cloud-init to fix the bug [0].
Related fix was proposed to upstream and recently was merged [1].
Now it's safe to remove all packages built from cloud-init sources [2].

After a bit of testing we also need to remove its git repository [3], and update jeepeb config and zuul pipelines accordingly.

List of packages to remove:
* Source: cloud-init
* Package: cloud-init
* Package: grub-legacy-ec2
* Package: ec2-init

[0] - https://bugs.launchpad.net/cloud-init/+bug/1597699
[1] - http://packages.ubuntu.com/xenial-updates/cloud-init
[2] - http://mirror.fuel-infra.org/mos-repos/xenial/master/pool/main/c/cloud-init/
[3] - https://review.fuel-infra.org/#/admin/projects/packages/xenial/cloud-init