Fix recompilation of htools on regen-vcs-version

Currently, most htools code depends on Constants.hs which is generated
from constants.py and also depends on _autoconf.py. Also, _autoconf.py
depends on vcs-version, which all together means that when 'make
regen-vcs-version' is run, for example by ./devel/upload, most of the
Haskell code needs recompilation.

Since htools already has its 'optimised' vcs-version (and doesn't use
the _autoconf.VCS_VERSION constants), we can optimise this as follows:

- _autoconf.py doesn't contain the VCS_VERSION anymore, and that is
  instead moved to _vcsversion.py
- constants.py depends on and imports this new module
- _autoconf.py doesn't get regenerated at vcs-version changes, but
  only at re-running configure/changing Makefile time

The end result is that only htools/Ganeti/HTools/Version.hs is
recompiled now, which is a significant speedup (usually < 1 second
versus 10 seconds previously).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>