pylint: enable `duplicate-key` check

pylint: enable `duplicate-key` check

This check attempts to detect dictionary literals with duplicate keys.

The rest of the Neutron tree has already had the few instances of this
cleaned[1] or moved into external vendor repos.  Enabling the pylint
check will stop future occurrences.

[1] I29cd2b843a7905986de13a1ecfba0cb5797ccaf8

(Original patch I1aa221d2019853f905f2b8421dd45b0a3102baf0 by zhiyuan_cai)

Change-Id: If4fed9714cd7fa586845f21f8f56dde2645cc5e0
Co-Authored-By: zhiyuan_cai <luckyvega.g@gmail.com>