lldp: add option/command to override hostname

[jonasj76]

Allow to override the system name (hostname) by using '-N' as parameter to
lldpd, or by using "configure system hostname " to lldpcli.

[vincentbernat]

Thanks for the patch! It looks good for me.

At some point in the future, I would like to remove most of the command line options in favor of the configuration files. I can merge your patch as is and remove the part adding the -N command-line flag or you can do it yourself or we can discuss why we should remove command line options (it's just that there are too many of them, now that we can use a configuration file, this seems redundant). Tell me what your prefer.

[jonasj76]

I agree about not adding more command-line options. I didn't need the command-line flag in the first place, it was just added to look like the option to override platform and system description.
I made another commit which removed the -N command flag.

[vincentbernat]

Thanks!