test(certificates): Add SSL certificate itests

[jan-law]

Related #474

Tests for null file uploads and conflicting file names that already exist in the truststore.

Question: If the /truststore directory is not mounted for integration tests, how can I test the following in CertificatePostHandler line 151?

if (fs.exists(filePath)) {
            throw new ApiException(409, filePath.toString() + " Certificate already exists");
        }

[andrewazores]

/truststore doesn't need to be mounted necessarily, like it's done in run.sh - this would allow the contents to be written to the host filesystem, but that's more than what's really needed for itest purposes. As long as the Cryostat instance under test is able to store the certificate for its single-run lifetime then that would be sufficient. So, a tmpfs mount would fit the bill. In pom.xml we do already mount tmpfs to some directories, ex:

              <argument>--mount</argument>
              <argument>type=tmpfs,target=/opt/cryostat.d/recordings.d</argument>

Adding another itest setup argument like this to mount the certificates storage directory (and possibly to set an environment variable telling Cryostat where that directory is within its container at runtime) should do the trick. After that you can test that the first POST succeeds and a subsequent identical POST fails with a 409.

[jan-law]

I've finished making changes, ready for review

[andrewazores]

Rebase please - I don't expect any conflicts between this commit and the last, but just to be sure.