Avoid recreate the VENV in setup-clienttools if it exists

This change prevent the setup-clienttools script from recreating
the VENV if one exists already (for example, if the script gets
re-runned).

Change-Id: I15322085f332fa493ad436b94b3e699eb9a2c9ac
Merge "Avoid recreate the VENV in setup-clienttools if it exists"