Add ability to set chassis management ip via LLDP plugin

[brunnels]

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Is your feature request related to a problem? Please describe.
Advertised LLDP address is usually the WAN IP address.

Describe the solution you'd like
I would like the ability to specify the interface to obtain the IP from or specify a specific IP address to use.

Describe alternatives you've considered
You can set it with lldpcli but it is not persistent and any modifications to /usr/local/etc/lldpd.conf will get reverted.

Additional context
You can set the management ip via lldpd.conf or lldpcli. Example: configure system ip management pattern 10.0.200.1