Allow keystoneclient to work with older keystone installs

Older keystone installs may return 501 for GET / instead of 300
like today. We should be able to assume that if 501 is returned the
server will support v2.0 only.

Fixes bug 1159911

Change-Id: If264840d8678a490264f1bdb62f1b51c362619e1
Merge "Allow keystoneclient to work with older keystone installs"