When [1] was committed, neutron stopped building for Train.
Analogous patch is proposed to other stable branches.
This patch removes neutron from upper-constraints.
Kolla master is affected directly because we build Train
for CentOS 7 atm.
1. https://review.opendev.org/#/c/697370/
Change-Id: I944e8e42fef1d359d767cbc6e1c13371ed753f31
121f294cff