Allow to manage default cert for custom TLS configurations

[jenskueper]

This PR allows to manage the default certificate for custom TLS configuration. This is relevant for PlatformTLS.

[kpfleming]

After some internal discussion it's going to be slightly challenging to accept this PR. While the code for the API endpoint is fine, the tests are not, because they will fail unless they are run using a user account that has one of the (limited) products enabled which support this field.

I think we'll need to have a separate set of test fixtures and test cases, which are skipped unless an environment variable named TEST_TLS_DEFAULT_CERTFICATE is set.

CC @Integralist

[Integralist]

The tests are run initially against the live API and those responses are saved into fixture files.

If the fixtures don't need updating any time soon then maybe it's fine. But yes, upgrading will require someone with access to this feature.