iotests: Check for quorum support in test 139

The quorum driver is always built in, but it is disabled during
run-time if there's no SHA256 support available (see commit e94867e).

This patch skips the quorum test in iotest 139 in that case.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Message-id: 1447172891-20410-1-git-send-email-berto@igalia.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
