swift: If storage_policy isn't set, use the root containers policy

Ensure that if we need to create a segments container it uses the same
storage policy as the root container.

Fixes #8858

fstest/testserver/init.d/TestSwiftAIO.d/remakerings

@@ -0,0 +1,33 @@
+#!/bin/bash
+
+cd /etc/swift
+
+if ! grep storage-policy swift.conf; then
+    cat <<EOF >> swift.conf
+
+# Policy-0 is the default, we need two policies to test policy inheritance.
+[storage-policy:0]
+name = Policy-0
+default = true
+
+[storage-policy:1]
+name = Policy-1
+EOF
+fi
+
+rm -f *.builder *.ring.gz backups/*.builder backups/*.ring.gz
+
+swift-ring-builder object.builder create 10 1 1
+swift-ring-builder object.builder add r1z1-127.0.0.1:6010/sdb1 1
+swift-ring-builder object.builder rebalance
+swift-ring-builder container.builder create 10 1 1
+swift-ring-builder container.builder add r1z1-127.0.0.1:6011/sdb1 1
+swift-ring-builder container.builder rebalance
+swift-ring-builder account.builder create 10 1 1
+swift-ring-builder account.builder add r1z1-127.0.0.1:6012/sdb1 1
+swift-ring-builder account.builder rebalance
+
+# For Policy-1:
+swift-ring-builder object-1.builder create 10 1 1
+swift-ring-builder object-1.builder add r1z1-127.0.0.1:6010/sdb1 1
+swift-ring-builder object-1.builder rebalance