Update load_minigraph to disable unnecessary services (bgp, lldp...) for fabric asic

[ngoc-do]

Fabric asics do not need some services like bgp, lldp, teamd.
They just need database, swss, and syncd running.

Signed-off-by: ngocdo ngocdo@arista.com

What I did

How I did it

I disable them with FEATURE flag configured in CONFIG_DB, which is listened and reacted by hostcfgd.

This PR will update CONFIG_DB's FEATURE for fabric asics. The update is executed in load_minigraph, and saved to config_db.json. So the update will run only once.

If a system has both NPU and fabric, only services running on fabric are disabled. NPU's services are not affected.

One field, always_disabled_on_asics is added to FEATURE's services that we want to disable for fabric asics. For example, bgp@4.service and bgp@5.service will be disabled.

    "FEATURE": {
        "bgp": {
            "state": "enabled",
            "has_timer": "False",
            "has_global_scope": "False",
            "has_per_asic_scope": "True",
            "auto_restart": "enabled",
            "high_mem_alert": "disabled",
            "always_disabled_on_asics": "4,5"
        },
   }

hostcfg will act on that change and disable the services. This is done in another PR at sonic-buildimage,
which is sonic-net/sonic-buildimage#7026.

How to verify it

Tested on our systems with NPU and fabrics.

Previous command output (if the output of a command-line utility has changed)

New command output (if the output of a command-line utility has changed)

[lguohan]

need unit test