tools/qa-checks: Enable default deletion of temporary Python virtual environment

This patch adds the --keep_venv option to run-checks.py. If this is not passed,
the temporary venv will be deleted after the script has been executed, to avoid
unexpectedly filling up the user's /tmp directory.

Issue-Id: SCM-3033
Signed-off-by: Richard Neill <richard.neill@arm.com>
Change-Id: Iaf6fcc3329d7608c5bc6b997a1de887638c64b2b