Configure apt and update index

This script configures apt to use a proxy (if VM_PROXY is set), adds the
ubuntu-cloud repo (release set by OPENSTACK_RELEASE) and updates the apt
index.

Partial-Bug: 1312764
Implements: blueprint openstack-training-labs
Change-Id: I6a426ee31746424a761fc8cd6d81723ac7740f04
Merge "Configure apt and update index"