Update `edge_port` role for the use with commercial parties

- PHY MTU - depends on the edge_port type (CUSTOMER, PUBLIC, etc)
- Add another hold-time map (it should be configured for PUBLIC
edge-ports)
- If `lacp_xmit_interval_slow` coming from GSO subscription is True,
then explicitely configure it (the default value for this in SROS is
`fast`).