Extend blackbox tests for encryption

[mulkieran]

I think this can go in a few steps. The general idea would be to work on the stratisd tests first, and then on the CLI tests.

For stratisd:

• Modify testlib/dbus.py to use the new interfaces. It is already using the new pool interface, and I don't think will need any new modifications once #506 is in. So, the only modification is to the manager interface. Hopefully, it will be possible here to keep the tests in stratisd_cert.py entirely the same.
• Extend the tests in stratisd_cert.py so that they also test encryption. This should be more challenging than the previous task, because it requires keyring management to make the tests run.
  The alternative is to prove that the testing infrastructure that the tests must run on will not support keyring management, and to give up entirely. We must check into this first.
• If it has been proven to be possible to do keyring management in the blackbox tests, then the CLI tests should also be extended for encryption.

Aside: if there is a global change to the test infrastructure, either for stratisd or for the CLI to make the tests have more useful message when they fail, that should be considered.

It would be quite suitable to do each of these changes in a separate PR.