Some env variable usage issues in scripts responsible for creating venv

[Ghanshyam Mann]

There are couple of issues in the scripts responsible for creating virtual environment.

1. install_venv.py
 - Tool path env variable is used as lower case in install_venv.py ('tools_path') and uppercase in other scripts with_venv.sh and run_tests_common.sh ('TOOLS_PATH')

we should use upper case ('TOOLS_PATH') in install_venv.py also.

2. with_venv.sh
- If some of env variable is being set not all then, it would not work as expected. for example if 'TOOLS_PATH' is only set then VENV path would not be proper.
 export TOOLS_PATH='/opt/stack/oslo-incubator/'
then venv gets created in '/opt/stack/oslo-incubator/.venv' but path constructed in with_venv.sh script will be '/opt/stack/oslo-incubator/../.venv' and it fails.

3. run_tests_common.sh
- VENV_NAME is being used to populate the VENV_DIR value but when venv name is being passed as argument in this script (--virtual-env-name) then VENV_NAME is not set and it fails with_venv.sh script which use VENV_NAME.

we should remove VENV_NAME and make use of VENV_DIR to fix this issue and to avoid confusion of having two env variable options.

[Davanum Srinivas (DIMS) (dims-v)]

Fixed in https://review.openstack.org/#/c/199432/