fixed AttributeError

fixed the below exception for few devices:

Error we are getting now:
  File "/Users/nitinkr/Documents/PyEZ/py-junos-eznc/lib/jnpr/junos/facts/swver.py", line 116, in facts_software_version
    f_master.lower())
AttributeError: 'list' object has no attribute 'lower'

If pull is accepted then only will add corresponding UT