use namespace argument in Pinecone constructor

[LeoGrin]

Fix #1756

Use the namespace argument of Pinecone.from_exisiting_index to set the default value of namespace for other methods. Leads to more expected behavior and easier integration in chains.

For the test, I've added a line to delete and rebuild the langchain-demo index at the beginning of the test. I'm not 100% sure if it's a good idea but it makes the test reproducible.