Modify Azure DNS Client logic to lazy load

Lazy loading the Azure DNS client ensures that the necessary network calls only occur when it is time to actually do something with the client.

octodns/provider/azuredns.py

@@ -323,6 +323,22 @@ class AzureProvider(BaseProvider):
     SUPPORTS = set(('A', 'AAAA', 'CAA', 'CNAME', 'MX', 'NS', 'PTR', 'SRV',
                     'TXT'))
 
+    def _get_dns_client(self):
+        if not self._dns_client_handle:
+            # Not initialized yet, we need to do that.
+            credentials = ServicePrincipalCredentials(
+                self._dns_client_client_id, 
+                secret=self._dns_client_key, 
+                tenant=self._dns_client_directory_id
+            )
+            self._dns_client_handle = DnsManagementClient(
+                credentials, 
+                self._dns_client_subscription_id
+            )
+            
+        return self._dns_client_handle
+            
+
     def __init__(self, id, client_id, key, directory_id, sub_id,
                  resource_group, *args, **kwargs):
         self.log = logging.getLogger('AzureProvider[{}]'.format(id))
@@ -330,10 +346,13 @@ class AzureProvider(BaseProvider):
                        'key=***, directory_id:%s', id, client_id, directory_id)
         super(AzureProvider, self).__init__(id, *args, **kwargs)
 
-        credentials = ServicePrincipalCredentials(
-            client_id, secret=key, tenant=directory_id
-        )
-        self._dns_client = DnsManagementClient(credentials, sub_id)
+        # Store necessary initialization params
+        self._dns_client_handle = None
+        self._dns_client_client_id = client_id
+        self._dns_client_key = key
+        self._dns_client_directory_id = directory_id
+        self._dns_client_subscription_id = sub_id
+        self._dns_client = property(_get_dns_client, _set_dns_client)
         self._resource_group = resource_group
         self._azure_zones = set()