[AIRFLOW-2140] Don't require kubernetes for the SparkSubmit hook (apache#3700)

This extra dep is a quasi-breaking change when upgrading - previously
there were no deps outside of Airflow itself for this hook. Importing
the k8s libs breaks installs that aren't also using Kubernetes.

This makes the dep optional for anyone who doesn't explicitly use the
functionality

Author: ashb

airflow/contrib/hooks/spark_submit_hook.py

@@ -26,7 +26,6 @@
 from airflow.exceptions import AirflowException
 from airflow.utils.log.logging_mixin import LoggingMixin
 from airflow.contrib.kubernetes import kube_client
-from kubernetes.client.rest import ApiException
 
 
 class SparkSubmitHook(BaseHook, LoggingMixin):
@@ -136,6 +135,10 @@ def __init__(self,
         self._connection = self._resolve_connection()
         self._is_yarn = 'yarn' in self._connection['master']
         self._is_kubernetes = 'k8s' in self._connection['master']
+        if self._is_kubernetes and kube_client is None:
+            raise RuntimeError(
+                "{master} specified by kubernetes dependencies are not installed!".format(
+                    self._connection['master']))
 
         self._should_track_driver_status = self._resolve_should_track_driver_status()
         self._driver_id = None
@@ -559,6 +562,6 @@ def on_kill(self):
 
                     self.log.info("Spark on K8s killed with response: %s", api_response)
 
-                except ApiException as e:
+                except kube_client.ApiException as e:
                     self.log.info("Exception when attempting to kill Spark on K8s:")
                     self.log.exception(e)

airflow/contrib/kubernetes/kube_client.py

@@ -17,9 +17,21 @@
 from airflow.configuration import conf
 from six import PY2
 
+try:
+    from kubernetes import config, client
+    from kubernetes.client.rest import ApiException
+    has_kubernetes = True
+except ImportError as e:
+    # We need an exception class to be able to use it in ``except`` elsewhere
+    # in the code base
+    ApiException = BaseException
+    has_kubernetes = False
+    _import_err = e
+
 
 def _load_kube_config(in_cluster, cluster_context, config_file):
-    from kubernetes import config, client
+    if not has_kubernetes:
+        raise _import_err
     if in_cluster:
         config.load_incluster_config()
     else: