session: Do not use MD5 in FIPS mode

Do not use MD5 when generating fingerprints in FIPS mode. The call will
fail in such case. The test suite was updated with a negative test for
this case.

Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>
Reviewed-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>