CI: deploy-cache: Store and reuse deploy artifacts

CI: deploy-cache: Store and reuse deploy artifacts

Add support for caching deploy artifacts, like bootstraps and
target images, which take a lot of time at each deploy to be built,
considering it requires a cross-debootstrap via qemu-user-static and
binfmt.

For OPNFV CI, the cache will piggy back on the <iso_mount> mechanism,
and be located at:
/iso_mount/opnfv_ci/<branch>/deploy-cache

TODO: Use dea interface adapter in target images fingerprinting.
TODO: remote fingerprinting
TODO: differentiate between bootstraps and targetimages, so we don't
end up trying to use one cache artifact type as the other.

FIXME: This is still a bit hacky in some areas, but we need it ASAP.

JIRA: ARMBAND-172

Change-Id: Iecd293db98c061325643cc61863a48f092bfd491
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>