esrf_undulator bug on config reading if using "undu_prefix"

esrf_undulator bug on config reading if using "undu_prefix"

CYRIL [4]: u32a.controller.initialize_axis(u32a)
!!! === ValueError: 'positio' is not in list === !!! 

CYRIL [4]: last_error
  Out [4]: 01/07/2021 10:55:10 Traceback (most recent call last):
             File "<stdin>", line 1, in <module>
             File "/users/blissadm/local/bliss.git/bliss/controllers/motors/esrf_undulator.py", 
                                                    line 143, in initialize_axis
               undulator_index = uname_list.index(uname)
           ValueError: 'positio' is not in list