add get_cluster_relations to api client

looks like:
```
In [1]: from hypernode_api_python.client import HypernodeAPIPython

In [2]: import os

In [3]: client = HypernodeAPIPython(os.environ['MYSECRETAPITOKEN'])

In [4]: client.get_cluster_relations("mytestappweb").json()
Out[4]:
{'parents': [{'id': 182,
   'parent': 'mytestappdb',
   'child': 'mytestappweb',
   'relation_type': 'mysql',
   'cluster_description': None},
  {'id': 180,
   'parent': 'mytestapp',
   'child': 'mytestappweb',
   'relation_type': 'loadbalancer',
   'cluster_description': None},
  {'id': 181,
   'parent': 'mytestapp',
   'child': 'mytestappweb',
   'relation_type': 'nfs',
   'cluster_description': None}],
 'children': []}
```

Author: vdloo

hypernode_api_python/client.py

@@ -5,6 +5,7 @@
 HYPERNODE_API_ADDON_SLA_LIST_ENDPOINT = "/v2/addon/slas/"
 HYPERNODE_API_APP_CHECK_PAYMENT_INFORMATION = "/v2/app/{}/check-payment-information/"
 HYPERNODE_API_APP_CONFIGURATION_ENDPOINT = "/v2/configuration/"
+HYPERNODE_API_APP_CLUSTER_RELATIONS = "/v2/app/{}/relations/"
 HYPERNODE_API_APP_DETAIL_ENDPOINT = "/v2/app/{}/?destroyed=false"
 HYPERNODE_API_APP_DETAIL_WITH_ADDONS_ENDPOINT = "/v2/app/{}/with_addons?destroyed=false"
 HYPERNODE_API_APP_EAV_DESCRIPTION_ENDPOINT = "/v2/app/eav_descriptions/"
@@ -365,6 +366,34 @@ def get_app_configurations(self):
         """
         return self.requests("GET", HYPERNODE_API_APP_CONFIGURATION_ENDPOINT)
 
+    def get_cluster_relations(self, app_name):
+        """ "
+        List all relations for the specified app. This will return all the
+        relations that are currently configured for the specified app.
+
+        Example:
+        >    client.get_cluster_relations('mytestappweb').json()
+        >    {'children': [],
+        >     'parents': [{'child': 'mytestappweb',
+        >                  'cluster_description': None,
+        >                  'id': 182,
+        >                  'parent': 'mytestappdb',
+        >                  'relation_type': 'mysql'},
+        >                 {'child': 'mytestappweb',
+        >                  'cluster_description': None,
+        >                  'id': 180,
+        >                  'parent': 'mytestapp',
+        >                  'relation_type': 'loadbalancer'},
+        >                 {'child': 'mytestappweb',
+        >                  'cluster_description': None,
+        >                  'id': 181,
+        >                  'parent': 'mytestapp',
+        >                  'relation_type': 'nfs'}]}
+        """
+        return self.requests(
+            "GET", HYPERNODE_API_APP_CLUSTER_RELATIONS.format(app_name)
+        )
+
     def get_product_info_with_price(self, product_code, error_to_raise=None):
         """
         Get information about a specific product

tests/client/test_get_cluster_relations.py

@@ -0,0 +1,28 @@
+from unittest.mock import Mock
+
+from tests.testcase import TestCase
+from hypernode_api_python.client import (
+    HypernodeAPIPython,
+    HYPERNODE_API_APP_CLUSTER_RELATIONS,
+)
+
+
+class TestGetClusterRelations(TestCase):
+    def setUp(self):
+        self.mock_request = Mock()
+        self.client = HypernodeAPIPython(token="mytoken")
+        self.client.requests = self.mock_request
+
+    def test_calls_hypernode_api_cluster_relations_endpoint_with_correct_parameters(
+        self,
+    ):
+        self.client.get_cluster_relations("yourhypernodeappname")
+
+        self.mock_request.assert_called_once_with(
+            "GET", HYPERNODE_API_APP_CLUSTER_RELATIONS.format("yourhypernodeappname")
+        )
+
+    def test_returns_result_for_hypernode_api_cluster_relations(self):
+        ret = self.client.get_cluster_relations("yourhypernodeappname")
+
+        self.assertEqual(ret, self.mock_request.return_value)