lag MTU setting fix - fix for #18695

[matiAlfaro]

What I did

Fix for #18695

How I did it

When creating PortChannel, don't set MTU MTU is set according to first port inserted to PortChannel. When last port is removed, set PortChannel MTU back to None

How to verify it

Tested with new test added to UT

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

Before this fix, you were not able to add to PortChannel a port with MTU !- 9100 (hard coded)

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

PortChannel MTU will be determined according to first port inserted into PortChannel