Simplify the structure of the grass init module

[pmav99]

This series of commits are all applied to lib/init/grass.py. The main goal is to simplify the structure of the module by moving everything that is related to the actual initialization process from the module level to main() but there are improvements as well (e.g. code simplifications, PEP8 etc)

The changes have been tested on:

• Linux x64 + python 2
• Linux x64 + python 3

If required, I can make a new PR with a single squashed commit, but I think that reviewing the changes, will be easier here.