Fix import for install_venv.py

On systems that don't use devstack the nova directory
doesn't automatically get added to the python path. This
breaks tools/install_venv_common.py which imports from
nova.openstack.common. This fixes the issue by manually
adding the nova root directory to the python path before
tools/install_venv_common.py is imported.

Change-Id: I8a3df02a51c2c777d8f47cb8bc72d0644ae47095
Merge "Fix import for install_venv.py"