Improve cold start time by moving kubernetes import

"import kubernetes" takes a long time. Moving this import improves
"tutor --version" from 0.8s to 0.3s.

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## Latest
 
+- [Improvement] Improve CLI cold start performance
 - [Improvement] Allow uppercase "Y" and "N" as answers to boolean questions
 
 ## v3.3.4 (2019-04-09)

tutor/k8s.py

@@ -1,5 +1,4 @@
 import click
-import kubernetes
 
 from . import config as tutor_config
 from . import env as tutor_env
@@ -138,6 +137,8 @@ class K8s:
     @property
     def client(self):
         if self.CLIENT is None:
+            # Import moved here for performance reasons
+            import kubernetes
             kubernetes.config.load_kube_config()
             self.CLIENT = kubernetes.client.CoreV1Api()
         return self.CLIENT