dpkg: local cache for .deb files

With this patch, /var/cache/apt/archives directory content is preserved.
The directory is actually a bind mount of the
~/.cache/image-create/apt/$DISTRO_NAME directory, much like what we do
for ccache.
You can use DIB_APT_LOCAL_CACHE=0 to disable this behavior.

This trivial change improve performance A LOT (>30%), even if a local
HTTP proxy because:
 - there is no need to copy again and again the same files
 - we avoid network latency

The patch has been tested with Debian and Ubuntu with every elements
from the tripleo-image-elements repository, the final size of the cache
directory is about 700MB per distribution subdirectory.

Change-Id: I4fab499493f734c7c546d4d23b1a98f0e7523a39
Do not use DatasourceNone for precise cloud-init

The 'None' Datasource does not exist in precise 12.04 (which uses
cloud-init 0.6.3), so conditionally ensure that we are not including
this in the datasource_list in case we're building a precise image.

Partial-bug: 1329068
Change-Id: I6efb91e17bf896bbf7d67c31c4eb5098d5d61a71
Merge "dpkg: local cache for .deb files"
Merge "Do not use DatasourceNone for precise cloud-init"