Fixed luks_add_key()
    
    On F18 with Anaconda 18.6.5, adding new passphrase to LUKS format always fails:
    
    ----> 1 fmt.addPassphrase("XYZ")
    
    /usr/lib64/python2.7/site-packages/pyanaconda/storage/formats/luks.pyc in addPassphrase(self, passphrase)
        246                           passphrase=self.__passphrase,
        247                           key_file=self._key_file,
    --> 248                           new_passphrase=passphrase)
        249
        250     def removeKeyFromFile(self, keyfile):
    
    /usr/lib64/python2.7/site-packages/pyanaconda/storage/devicelibs/crypto.pyc in luks_add_key(device, new_passphrase, passphrase, key_file)
        133
        134     cs = CryptSetup(device=device, yesDialog = askyes, logFunc = dolog, passwordDialog = askpassphrase)
    --> 135     rc = cs.addPassphrase(passphrase = passphrase, newPassphrase = new_passphrase)
        136
        137     if rc<0:
    
    In recent cryptsetup-python-1.5.0-2.fc18, cs.addPassphrase was replaced
    probably by cs.addKeyByPassphrase (I'm just guessing by the name). I tested it
    just in my dummy applications, take it with a grain of salt.
    
    Signed-off-by: Jan Safranek <jsafrane@redhat.com>