Nessus report downloads can include characters that cause the nessus_download_all script to break

[JPvRiel]

For the nessus_download_all script/example, a report name containing report/report breaks.

Traceback (most recent call last):
  File "./nessus_download_all.py", line 76, in <module>
    fp = open('%s_%s.nessus'%(scanner.scan_name,scanner.scan_id),'w')
FileNotFoundError: [Errno 2] No such file or directory: 'report/report'