Wltest setup bugs

Created by: qperret

When setting up wltest for the first time we noticed that the following problems are not caught:

• it is possible to run lisa-wltest-series without having the virtualenv binary installed which leads to all sorts of errors. The presence of virtualenv should be checked in lisa-wltest-init
• when there are permissions issues with fastboot (udev rules not set correctly, sudo rights needed, ...) the user will face the "timeout to reboot in bootloader" error. This should be caught with a better error message at least.