mongodb: install python-pymongo for monitoring

Backport from commit 0858f53b2a70718d8b1f4567a4348c3e9b5c4faf

mongodb/tasks/main_jessie.yml

@@ -17,6 +17,11 @@
     name: mongodb-org
     state: installed
 
+- name: install dependency for monitoring
+  apt:
+    name: python-pymongo
+    state: installed
+
 - name: Custom configuration
   template:
     src: mongod_jessie.conf.j2

mongodb/tasks/main_stretch.yml

@@ -8,6 +8,11 @@
     - mongodb
     - mongo-tools
 
+- name: install dependency for monitoring
+  apt:
+    name: python-pymongo
+    state: installed
+
 - name: Custom configuration
   template:
     src: mongodb_stretch.conf.j2