Fix problem with Ceilometer Swift pollsters for RadosGW

When RadosGW is used as Swift backend, Swift pollsters
didn't work, because RadosGW has different endpoint
from native Swift. In this case, Ceilometer should check
parameter 'swift_rados_backend' and choose correct endpoint

Change-Id: If27a62da8f849868848a4e7ab62a275b0a0d323d
Closes-Bug: #1352861
Merge "Fix problem with Ceilometer Swift pollsters for RadosGW"