Test for pip became main check for bootstrap installation

[vitkhab]

For first time installing bootstrap I forgot to define variable "ansible_python_interpreter". Bootstrap didn't install pip. On repeated runs all checks were skipped.

So I made pip check to be main test for bootstrap installation. All other checks are skipped if pip check is successful.

[defunctzombie]

You can't use python before bootstrap since it won't be available yet as far as I am aware.

[vitkhab]

Error will be generated if bootstrap is not installed (so python is not available) or pip is not installed. In both cases bootstrap installation will be tested and if it's not already installed, it will be. If it is installed, then only pip installation will be processed.