jepsen: fix race condition with apt's sources.list

Jepsen testing starts one or several virtual machines and runs tarantool
instances on them. The first (first important for us here) command on
the virtual machine is `apt-get <...> update`: we should download
packages list to allow Jepsen to install necessary dependencies.

However we can access the virtual machine (using ssh) before it is fully
initialized by the cloud-init script. In particular, the cloud-init
script replaces apt's mirror list file (`/etc/apt/sources.list`).

Normally we should call `apt-get <...> update` after the package list
update, but here cloud-init races with the update command.

In the bad case the commands are executed in the opposite order:

* Terraform calls `apt-get <...> update`.
* cloud-init replaces `/etc/apt/sources.list`.

Now an attempt to install a package using apt-get will give the 'unable
to locate package' error, because we have no packages list for the 'new'
mirrors.

The problem is nicely described in [1]. See also the linked issue for
details.

[1]: https://github.com/hashicorp/packer/issues/41#issuecomment-21288589

Fixes https://github.com/tarantool/jepsen.tarantool/issues/87

extra/tf/main.tf

@@ -28,6 +28,7 @@ resource "openstack_compute_instance_v2" "instance" {
   provisioner "remote-exec" {
     inline = [
       "set -o errexit",
+      "sudo cloud-init status --wait",
       "sudo hostnamectl set-hostname n${count.index + 1}",
       "sudo apt-get -o Debug::Acquire::http=true -o Debug::pkgAcquire::Worker=1 update"
     ]