pkg/azure: fix data race in shared field
pkg/azure: fix data race in shared field

Fixes the following race:

```
WARNING: DATA RACE
Read at 0x00c0002fe870 by goroutine 460:
  github.com/cilium/cilium/pkg/azure/ipam.(*Node).ResyncInterfacesAndIPs()
      /home/travis/gopath/src/github.com/cilium/cilium/pkg/azure/ipam/node.go:162 +0x1a4
  github.com/cilium/cilium/pkg/ipam.(*Node).recalculate()
      /home/travis/gopath/src/github.com/cilium/cilium/pkg/ipam/node.go:357 +0x12b
  github.com/cilium/cilium/pkg/ipam.(*NodeManager).resyncNode()
      /home/travis/gopath/src/github.com/cilium/cilium/pkg/ipam/node_manager.go:384 +0x92
  github.com/cilium/cilium/pkg/ipam.(*NodeManager).Resync.func1()
      /home/travis/gopath/src/github.com/cilium/cilium/pkg/ipam/node_manager.go:431 +0xa0
Previous write at 0x00c0002fe870 by goroutine 57:
  github.com/cilium/cilium/pkg/azure/ipam.(*InstancesManager).Resync()
      /home/travis/gopath/src/github.com/cilium/cilium/pkg/azure/ipam/instances.go:105 +0x8f9
  github.com/cilium/cilium/pkg/ipam.(*NodeManager).instancesAPIResync()
      /home/travis/gopath/src/github.com/cilium/cilium/pkg/ipam/node_manager.go:186 +0x8b
  github.com/cilium/cilium/pkg/ipam.NewNodeManager.func1()
      /home/travis/gopath/src/github.com/cilium/cilium/pkg/ipam/node_manager.go:168 +0x8f
  github.com/cilium/cilium/pkg/trigger.(*Trigger).waiter()
      /home/travis/gopath/src/github.com/cilium/cilium/pkg/trigger/trigger.go:206 +0x5b1
```

Fixes: 3dfd638dcf7b ("ipam: Move iterator logic into generic InstanceMap")
Signed-off-by: André Martins <bc9800b9d52a24cce72a73dd528afed53f10e5fc@cilium.io>

tgraf/cilium,tgraf/cilium,tgraf/cilium,tklauser/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,tklauser/cilium,tklauser/cilium,cilium-team/cilium,tgraf/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,cilium/cilium,cilium-team/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium