Fix Vagrantfile to append to authorized keys instead of overwrite
Add names to the different feature vectors so they are easier to read
Add warning in README about default SSH key
Remove user._profile_cache when trying to connect to a server

This causes problems if the user updates their ssh keys, they will
no longer be able to connect because the previous keys were
already cached.
Add test case for updating ssh keys
Change test_change_ssh_key to only use the user without a credential