Add 'ca' plugin
    
    This commit adds the 'ca' plugin for creating and managing
    lightweight CAs.  The initial implementation supports a single level
    of sub-CAs underneath the IPA CA.
    
    This commit also:
    
    - adds the container for FreeIPA CA objects
    
    - adds schema for the FreeIPA CA objects
    
    - updates ipa-pki-proxy.conf to allow access to the Dogtag
      lightweight CAs REST API.
    
    Part of: https://fedorahosted.org/freeipa/ticket/4559
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>