topotests: precise importation folder

topotests: precise importation folder

the topolog importation folder must be precised. otherwise following
error message appears:

root@dut-vm:~/topotests/bgp_flowspec# python3 test_bgp_flowspec_topo.py
Traceback (most recent call last):
  File "test_bgp_flowspec_topo.py", line 96, in <module>
    from lib.lutil import lUtil
  File "/root/topotests/bgp_flowspec/../lib/lutil.py", line 25, in <module>
    from topolog import logger
ImportError: No module named 'topolog'
root@dut-vm:~/topotests/bgp_flowspec#

The same error occurs with lutil and bgprib which are 2 libraries
located under lib/ folder. Some precisions are added too.

PR=71290
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>