New certificates are added without creating files

LeapHttpClient can fetch the main CA certificate downloaded from the
provider and add it to its in-memory keystore, without saving the
certificate in a file.

This is a very important feature, I think it can be cherry picked (it has
little modifications, and very localized).
It's very important because authentication does not work without this
bug fix in the latest branches, because I removed the code that saved
the certificate in a file but didn't test the authentication part.