Identifying CAs provided by backend plugins.

It is possible to have multiple CA plugins, each potentially talking to
multiple backend CA servers. A mechanism is therefore needed to allow
the client to select a backend CA server.

In addition, Dogtag plans to implement the ability to configure lightweight
sub CA's - subordinate CA's that can exist within the same CA instance. This
opens up the possibility of configuring a separate CA instance for each
project, so that the project could have certificates that are scoped to the
project only. Thus, a mechanism is also required to associate a project with
a preferred CA, so that if a client does not request a specific CA, the
preferred CA is selected.

Also, a mechanism should be added to allow clients to discover the CA servers
available for a particular Barbican instance.