Enable selection between Python and Haskell confd

This patch changes configure.ac Makefile.am so that the user can pass:

- --disable-confd (or --enable-confd=no) to disable it completely
- --enable-confd=yes or --enable-confd=python to select the
  traditional implementation (this is the default setting)
- --enable-confd=haskell to select hconfd

The only "not nice" thing is that I've chosen to keep the
hconfd.hs/hconfd name, and we rename it after install via an
install-exec-hook. The other choice is possible too (to rename the
source file/binary).

One additional note is that if we select haskell, the _rule_ for
creating daemons/ganeti-confd dissapears; whereas if we select python,
the rule for htools/hconfd still exists (one can build it explicitly),
it just is not installed. This is due to the different way in which
the rules are declared.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>