junos interfaces parser: do not propagate interface descr to subif

[ddutt]

we were incorrectly setting the description of Junos subinterfaces to be that of the interface. Subinterfaces can have their own descriptions. Use subinterface description if provided.

New Behavior

Subinterface descriptions are correctly extracted for Junos devices.

Proposed Release Note Entry

Correctly extract subinterface descriptions for Junos devices.

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