Corrects a flaw in the treatment of swift endpoints

When keystone has a registered swift endpoint of which the url does not start
with '/v1', the object-store pollster is not able to use it correctly.
This commit fixes it's behaviour to provide more flexibility in endpoint urls.

Change-Id: I579b1cf450f16853ac49f1d248a64a179e6443d9
Closes-Bug: #1325591
Merge "Corrects a flaw in the treatment of swift endpoints"