Remove -U from pip install
Remove -U from pip install

'pip install -U' ugrades specified packages, this is not necessary
since we use constraints, remove the parameter '-U' from the line.

With tools/tox_install.sh - which a previous change of mine removed -
the -U was not harmful, but with the current set up, it might cause
upgrades, so remove it.

Change-Id: I49dfbdcf72f0322b9b20ac2294e57556a4ef6441

openstack/oslo.reports