new commands to manage user/host/service certificates
    
    A new group of commands is introduced that simplifies adding and removing
    binary certificates to entries. A general form of the command is
    
    ipa [user/host/service]-[add/remove]-cert [pkey] --certificate=[BASE64 BLOB]
    
    Part of http://www.freeipa.org/page/V4/User_Certificates and
    https://fedorahosted.org/freeipa/ticket/4238
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>