Move hash functions to the compat module

Move hash functions to the compat module

Since the hash functions' changed their module name between python 2.4
and 2.6, and we have to do an try/import/except trick, we'll do it just
once, for both hash functions, and in compat.py. This also fixes a use
of md5 in the utils unittests which didn't use the trick before, and
generated a deprecation warning under 2.6.

In compat we keep both a ganeti-wide non-version-specific version to be
used by other ganeti modules, and a python-version specific that can be
passed to python modules which expect a hash function for their input
but call it differently under different versions of python (hmac, for
example).

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