ipam/host-local: add ResolvConf argument for DNS configuration

This adds the option `resolvConf` to the host-local IPAM configuration.
If specified, the plugin will try to parse the file as a resolv.conf(5)
type file and return it in the DNS response.

plugins/ipam/host-local/backend/allocator/config.go

@@ -32,6 +32,7 @@ type IPAMConfig struct {
 	Gateway    net.IP        `json:"gateway"`
 	Routes     []types.Route `json:"routes"`
 	DataDir    string        `json:"dataDir"`
+	ResolvConf string        `json:"resolvConf"`
 	Args       *IPAMArgs     `json:"-"`
 }