Adding a "bind to address" to the bootstrap proto (#136)

For envoy/#1411

Adding it in a structured proto because in the long run I anticipate us wanting to have per-cluster port ranges we bind to and it makes sense to me if we're going to eventually allow the bootstrap and cluster config to interact that we stick all the interacting bits in another message.

api/bootstrap.proto

@@ -6,9 +6,17 @@ syntax = "proto3";
 
 package envoy.api.v2;
 
+import "api/address.proto";
 import "api/base.proto";
 import "api/cds.proto";
 
+// An extensible structure containing the address Envoy should bind to when establishing upstream
+// connections.
+message UpstreamBindConfig {
+  // The address Envoy should bind to when establishing upstream connections.
+  ResolvedAddress source_address = 1;
+}
+
 message Bootstrap {
   // Node identity to present to the management server and for instance
   // identification purposes (e.g. in generated headers).
@@ -25,5 +33,7 @@ message Bootstrap {
   // to know how to speak to the management server. These cluster definitions
   // may not use EDS (i.e. they should be static IP or DNS-based).
   repeated Cluster bootstrap_clusters = 5;
+  // Optional configuration used to bind newly established upstream connections.
+  UpstreamBindConfig upstream_bind_config = 6;
   // TODO(htuch): Add support for HDS.
 }