Create functional package upgrade tests

There should be tests to ensure that upgrades to new MaxScale packages work as expected (see MXS-49 and MXS-50 for why this is important).

Here are some ideas about what should be tested:

1. Make sure the upgrade is successful
2. Make sure the service starts after upgrade
3. Make sure the service stops after upgrade
4. Make sure the new package can be removed without problem
5. Make sure the new package can be installed again without problem
6. Make sure the package can be downgraded to the previous release

Maybe there are more?