Replace arp-scan with nmap network scanner

Now we use the nmap scanner for arp scanning with
parameters "-n -sn -sP". To execute nmap with the root privilege it is
used the sudo command. Since the sudoers confirugarion should allow to
run the nmap command without password prompt.
Restrict line length according to PEP8
Restrict line length in settings files
Remove tail spaces
Now imports are by one in the line
Write regular expressions using one line
Fix bug in the get_port_value function
Change a log message in check_settings function