Euca 7893

When authentication expires (independent of session), renew authentication. Works for EUCA, not AWS since we don't keep enough info on the proxy to re-authenticate to AWS.

This may seem like a lot of code changes, but it is pretty isolated.
- add password to user session so we can re-auth later
- trap auth errors in cache refresh and regular API callbacks, then call method to renew creds
- new method to renew creds (in session.py) which gets new creds, then ensures they are passed around to places that need them.