* Fix Intel EK certificate URL
To download the certificate for an Intel TPM, the base64 padding
in the URL needs to be replaced with `%3D`. If it's not replaced,
requesting the URL will result in HTTP 403 Forbidden.
* Use `url.QueryEscape` to escape base64 padding
This PR adds:
* Renames 'PlatformEK' to 'EK'
* More consistant support of EKs without certificates
* Removes HTTP GET to Intel EK certificate service
* Always populates EK.Public
