tox: shorter envdir name for upgrade target
tox: shorter envdir name for upgrade target

The shebang of a script in linux is limited to 127 chars (execve
limitation). In gate, within the virtualenv the python path in all
binaries shebang is a bit long:

/home/jenkins/workspace/gate-gnocchi-tox-db-py35-postgresql-file-upgrade-from-2.2-ubuntu-xenial/.tox/py35-postgresql-file-upgrade-from-2.2/bin/python

This change sets a envdir manually to workaround the issue and keep
explicit name in tox target.

Change-Id: I6edadc5e03891595db23afc12ee36a519989bd8a

leandroreox/gnocchi,gnocchixyz/gnocchi,sileht/gnocchi,sileht/gnocchi,leandroreox/gnocchi,gnocchixyz/gnocchi