Update ros.py to support CHR

[nunomcruz]

Previous implementation supposed there was always a routerboard method, however this isn't true for CHR, where a serial number isn't also available.

[luqasz]

Hi.

Sorry for my delayed reply. I will look at this closer in few days.

[luqasz]

@nunomcruz Please rebase. I'fe fixed get_interfaces() error.

[luqasz]

Please update to newest master. I've fixed pylint errors.

[stale[bot]]

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