Fix dsvm-functional and cleanup tox.ini
Fix dsvm-functional and cleanup tox.ini

Remove unnecessary things from tox.ini:
- install_command: it's better to use the built-in install command
- VIRTUAL_ENV: this is set anyways
- sitepackages: it's enough to use virtualenv's packages

Besides these seem to be unnecessary 'sitepackages' setting started to
break functional job with an error message that there are missing
packages, however those were installed. By removing sitepackages=True
setting the errors dissapear.

Closes-Bug: #1876028
Change-Id: Ifc4904fa89eb7fcaaa8cd623112f419ac57ea286

openstack/networking-bgpvpn,openstack/networking-bgpvpn,openstack/networking-bgpvpn