Refs #6875 - separate the default CA and server CA

Up until now, we used the default CA for both server and client certificates.
This made practically impossible to issue the server certificates outside of
the installer and pass it in as arguments.

By default, the server CA is the same as default CA, unless the $server_ca_cert
is specified.

In the bootstrap rpm, we ship both server CA (for verifying the server) as well
the default CA (for verifying the qpid by the gofer).