Respect in-project config even when .venv exists

Before this change, when .venv directory exists  within the project 
root, poetry will use it as the path to the venv regardless whether 
virtualenvs.in-project is set to true or not. This leads to confusion
as described in #1770 and #2756.

With this change poetry will check if virtualenvs.in-project is set to
true if it finds a .venv folder.

This commit also changes the default state of this configuration to be
unset (null), in order to not break any current environments.

Resolves: #1770 #2756