use namespace argument in Pinecone constructor
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.