add test for external CA key size sanity check
We recently added validation of externally-signed CA certificate to
ensure certificates signed by external CAs with too-small keys
(according to system crypto policy) are rejected.
Add an integration test that attempts to renew with a 1024-bit
external CA, and asserts failure.
Part of: https://pagure.io/freeipa/issue/7761
Reviewed-By: Christian Heimes <email@example.com>