Set backend_alg for swift_proxy to source
Set backend_alg for swift_proxy to source

When making calls to swift-proxy in quick succession, and eventual
consistency, having "leastconn" as default for swift-proxy can cause
unexpected issues. Especially evident when running tempest on a
multinode install.

Change-Id: I5cc87c219dab0725eef7fb9a90420623a18496f8
Closes-Bug: #1428081

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