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: I9f818d4b78e7540498a1501be14cd63ac3e891b3

tox.ini

@@ -5,7 +5,7 @@ skipdist = True
 
 [testenv]
 usedevelop = True
-install_command = pip install -U {opts} {packages}
+install_command = pip install {opts} {packages}
 setenv = VIRTUAL_ENV={envdir}
          OS_STDOUT_CAPTURE=1
          OS_STDERR_CAPTURE=1