Pin tox to 2.7.0

There is a fun new bug [0] in tox 2.8.0 that affects tempest. To avoid
this crashing everything to a hault pin to 2.7.0 for now.

[0] https://github.com/tox-dev/tox/issues/595

Change-Id: I811fa4c71b63cfa39c3fd1018f9e3d3117ee2300

nodepool/elements/infra-package-needs/install.d/10-pip-packages

@@ -20,7 +20,7 @@ if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
 fi
 set -e
 
-packages='tox'
+packages='tox==2.7.0'
 
 for package in $packages ; do
     pip install $package