Allow configuration of session renewal.

Also set default timeout to 30 minutes.  This works around a situation
where we used an expired token.  I saw symptoms where CloudFiles
temporary signed URLs worked for hours 0-23, renewed the token, worked
for hours 23-24, then failed for hours 24-46.  The cycle then repeats.
Merge pull request #1415 from maginatics/reduce-keystone-auth-renewal

Reduce Keystone 1.1 auth token renewal timer