[dualtor] Add script to verify consistency between kernel and ASIC (#…

…2840)

#### What I did
Add script `dualtor_neighbor_check.py` to verify the neighbor consistency
based on the mux state. It will have the following output:
```
NEIGHBOR       MAC                PORT        MUX_STATE    IN_MUX_TOGGLE    NEIGHBOR_IN_ASIC    TUNNERL_IN_ASIC    HWSTATUS
-------------  -----------------  ----------  -----------  ---------------  ------------------  -----------------  ----------
192.168.0.2    ee:86:d8:46:7d:01  Ethernet4   standby      False            no                  yes                consistent
192.168.0.3    86:73:c2:22:bf:02  Ethernet8   standby      False            no                  yes                consistent
192.168.0.24   56:a6:bf:c5:dd:17  Ethernet92  active       False            yes                 no                 consistent
192.168.0.25   3a:18:56:f5:02:18  Ethernet96  active       False            yes                 no                 consistent
192.168.0.100  00:00:00:00:00:00  N/A         N/A          N/A              no                  yes                consistent
```

Signed-off-by: Longxiang Lyu <[email protected]>

#### How I did it
the workflow of this scripts:
1. for non-zero-mac neighbors in `APPL_DB` `NEIGH_TABLE`, use the `ASIC_DB` fdb entries to find the mux port that it belongs to.
2. check if the neighbor is consistent with mux state:
    * if mux state is `active`, the neighbor is consistent only if the neighbor is present in `ASIC_DB` but no tunnel route.
    * if mux state is `standby`, the neighbor is consistent only if the tunnel route is present in `ASIC_DB`1 but no neighbor.
3. if there are any inconsistent neighbors and the mux port is currently in-toggle, the script will have a non-zero negative return, and will write error messages to logs.


#### How to verify it
UT and verify on testbed.

#### 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)