Fix for delay flex counters flow

[shlomibitton]

Signed-off-by: Shlomi Bitton shlomibi@nvidia.com

What I did
Exclude installing flex counter for CPU interface (mgmt) for proper query counter capability.
Fixing this PR: #1646

Why I did it
The CPU interface does not suppose to be inserted to flex counter DB.
In addition, if we query the counter statistics capability by this interface it will cause all port counters id lists to be empty and break lua port rates functionality.

How I verified it
Enable all counters group types and observe all ID lists for ports populated correctly.

Details if related

[liat-grozovik]

missing unit test to make sure that when CPU port is the first it is working as expected. so then you can add the original fix which is now expected to be reverted.

[shlomibitton]

@qiluo-msft @lguohan Can you please review this?
This is the fix for the issue introduced from PR: #1646 #1736
I added another unit test to verify this case.
To merge this fix properly we need to merge again the previous PR's and then merge this fix to both branches.

[qiluo-msft]

Could you included previous code into this PR so we can merge this PR atomically?

[shlomibitton]

Could you included previous code into this PR so we can merge this PR atomically?

@qiluo-msft I created 2 new PR's with the previous code, issue fix and new unit test case for both branches.
master: #1749
202012: #1750

I am closing this PR since it is not relevant