ipautil: move file encryption functions to installutils
    
    The encrypt_file() and decrypt_file() functions depend on ipaplatform.
    
    Move them to ipaserver.install.installutils, as they are only used for the
    server installer.
    
    https://fedorahosted.org/freeipa/ticket/6474
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>