run_tests.sh fixup venv/novenv logic

run_tests.sh fixup venv/novenv logic

The section which prompts y/n for creating a new venv can never be
reached with any combination of CLI options (since always_venv=0 and
never_venv=0 can never happen simultaneously)

This actually makes the --no-virtual-env option pointless, so we may
as well remove it and explicitly set it as the default (it was anyway
previously but by virtue of the never_venv option being undefined)

Change-Id: I3c803533fda55a6814b7536b5c1328312acd3d57
Signed-off-by: Steven Hardy <shardy@redhat.com>