Introduce git reference/tag tracking for debugging

This patch adds a new vcs-version file that is generated via git (and
can be adapted if VCS is changed) and then embebbded as VCS_VERSION in
the constants module.

This means two things:
- local modifications without committing to git (or when using a tar.gz
  archive + mods) will not be reflected
- version is fixed at the time of the last make regen-vcs-version (dist time,
  or devel/upload which calls this)

Thus this is more geared at developers rather than end users.

The patch:

- adds rules for generating the vcs-version file
- adds a dist-hook for re-generating the file (if possible) and copying
  the updated version to the distdir
- modifies devel/upload to re-generate the file before upload

The output of --version will look like:
gnt-cluster (ganeti v2.2.0beta0-184-gebca7e6) 2.2.0~beta0

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>