ipam/host-local: support sets of disjoint ranges

In real-world address allocations, disjoint address ranges are common.
Therefore, the host-local allocator should support them.

This change still allows for multiple IPs in a single configuration, but
also allows for a "set of subnets."

Fixes: #45

plugins/main/bridge/bridge_test.go

@@ -94,14 +94,14 @@ const (
 	rangesStartStr = `,
         "ranges": [`
 	rangeSubnetConfStr = `
-            {
+            [{
                 "subnet":  "%s"
-            }`
+            }]`
 	rangeSubnetGWConfStr = `
-            {
+            [{
                 "subnet":  "%s",
                 "gateway": "%s"
-            }`
+            }]`
 	rangesEndStr = `
         ]`