Accept data plane key/cert directly (not only from file)

[thomasht86]

This is the current signature of Vespa()class:

 Vespa(url, port=None, deployment_message=None, cert=None, key=None, vespa_cloud_secret_token=None, output_file=sys.stdout, application_package=None, additional_headers=None)

We want to add key_content and cert_content parameters, which would allow user to read key/cert directly from for example .env file.
Providing these should be mutually exclusive with providing key/cert as path.
We want both unit tests and integration tests for this.