Tidy base image by removing /var/log files

diskimage-builder/elements/base/cleanup.d/99-tidy-logs added
to tidy up (remove) initial logs files on disk images.

Makes for slightly smaller images, and easier times tracing post
boot issues.

Change-Id: Iee98b5d818e080bcd9635482bc29349a71938087
Don't try to install if packages is empty

After running through pkg-map we could have no packages to install,
if so, don't attempt to run a malformed command.

Change-Id: Ia8e0aed62bcf814bf85c86b54ff0837da49ae7dd
Relabel filesystem if SELinux is available

Relabel the filesystem during image builds if SELinux is supported
in the kernel of the build machine and userspace tools are available.

Otherwise touch /.autorelabel to schedule a relabel the first time
the image boots. We relabel when possible because it decreases first
boot time.

Change-Id: I0bec885d6e5d4f4e1106f3bd2a90ba5f86395b07
Partial-Bug: 1347845
Merge "Tidy base image by removing /var/log files"
Merge "Relabel filesystem if SELinux is available"
Merge "Don't try to install if packages is empty"
Allow to specify an empty package list in pkg_map

An empty package list can be provided in a pkg_map file for those
cases were there is no need to install the package.

Change-Id: I0df8c6848ec4ac9e0f90790f14e8eb5399650339
Merge "Allow to specify an empty package list in pkg_map"
Increase source-repositories support for tarballs

The existing support for tar files in source-repositories saves
only the contents within subdirectories contained in the archive.

This change makes the selection of contents within the tarball user-
definable based on the REPOREF specified (or overridden in the env).

A REPOREF of '*' (the current default, but deprecated) will select
the contents of subdirectories within the archive, while '.' will
select the entire contents of the archive.

For reference:
http://lists.openstack.org/pipermail/openstack-dev/2014-August/043197.html
http://lists.openstack.org/pipermail/openstack-dev/2014-August/043249.html

Change-Id: I45db42ce66bf1d63d6ab5730090bf458b1b37ce9
Merge "Increase source-repositories support for tarballs"