Use set_setting in rax.py inventory module

Use `set_setting` in rax.py inventory plugin to set the `identity_type` to `rackspace`.  This will alleviate Pyrax's need to look for a `~/.pyrax.cfg` locally.

Oh, and a small correction to the documentation pointing out the need to use `RAX_CREDS_FILE` env var and not `RAX_CREDS`.