Azure: correct default tags value for VMs

In case tags are not supplied when the vm is created, the Azure
API will not return an entry for them.  Supply an appropriate
default value.

Change-Id: I036c3560903ae9a503fd5b9fdae44bcde175babc

nodepool/driver/azure/adapter.py

@@ -45,7 +45,7 @@ class AzureInstance(statemachine.Instance):
     def __init__(self, vm, nic=None, public_ipv4=None,
                  public_ipv6=None, sku=None):
         self.external_id = vm['name']
-        self.metadata = vm['tags'] or {}
+        self.metadata = vm.get('tags', {})
         self.private_ipv4 = None
         self.private_ipv6 = None
         self.public_ipv4 = None