fix variable names

playbooks/roles/mongo_client/defaults/main.yml

 ---
-MONGODB_APT_KEY: "7F0CEB10"
-MONGODB_APT_KEYSERVER: "keyserver.ubuntu.com"
-MONGODB_REPO: "deb http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.2 multiverse"
-mongo_version: 3.2.12
+MONGO_CLIENT_MONGODB_APT_KEY: "7F0CEB10"
+MONGO_CLIENT_MONGODB_APT_KEYSERVER: "keyserver.ubuntu.com"
+MONGO_CLIENT_MONGODB_REPO: "deb http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.2 multiverse"
+mongo_client_version: 3.2.12
 
 mongo_client_debian_pkgs:
-  - "mongodb-org-shell={{ mongo_version }}" 
-  - "mongodb-org-tools={{ mongo_version }}"
+  - "mongodb-org-shell={{ mongo_client_version }}" 
+  - "mongodb-org-tools={{ mongo_client_version }}"

playbooks/roles/mongo_client/tasks/main.yml

 ---
 - name: add the mongodb signing key
   apt_key:
-    id: "{{ MONGODB_APT_KEY }}"
-    keyserver: "{{ MONGODB_APT_KEYSERVER }}"
+    id: "{{ MONGO_CLIENT_MONGODB_APT_KEY }}"
+    keyserver: "{{ MONGO_CLIENT_MONGODB_APT_KEYSERVER }}"
     state: present
   tags:
     - install
@@ -10,7 +10,7 @@
 
 - name: add the mongodb repo to the sources list
   apt_repository:
-    repo: "{{ MONGODB_REPO }}"
+    repo: "{{ MONGO_CLIENT_MONGODB_REPO }}"
     state: present
   tags:
     - install