Don't delete test VMs by default.

We trash their content when we deploy to them - there's no need to
wipe the VM's out if someone runs devtest_setup by hand (and indeed
doing so would be surprising as devtest_testenv.sh is the thing that
makes test VMs, but might not be run by the user).

Change-Id: I2de229dbb51e25eeff5db40c4479b0928705eedf
Merge "Don't delete test VMs by default."