Add pre-{install,purge} support for base image

Add pre-{install,purge} support for base image

Extend <lib.sh> and its invocation from <ci.deploy.sh> with
support for modifying foundation node VMs base image prior to
using it with:
- additional APT GPG keys;
- additional APT repos;
- packages to pre-install;
- packages to pre-remove;
- (non-configurable) cloud init datasource via NoCloud only,
  so VCP VMs won't wait for metadata service;

While at it, re-use the resulting image as a base for another
round of pre-patching (same operations as above are supported)
to provide a base image for VCP VMs.

Add AArch64-specific configuration based on new mechanisms:
- pre-install linux-image-generic-hwe-16.04-edge (and headers)
  for foundation node and VCP (common) image (also requires new
  repo and its key);
- pre-install cloud-init for VCP image (it should already be
  installed, but script needs non-empty config for VCP to create
  the VCP image and transfer it over to Salt Master);

NOTE: cloud-init is required on VCP VMs for DHCP on 1st iface.

JIRA: FUEL-309

Change-Id: I7dcaf0ffd9c57009133c6d339496ec831ab14375
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>