Convert ELK repo location to a variable so it can be overridden

Change-Id: I9e5b78960c891aae7f4e94317647668a77b08c58

elk_metrics_6x/common_task_install_elk_repo.yml

@@ -24,7 +24,8 @@
     state: present
     update_cache: yes
 
-- name: add metricsbeat repo to apt sources list
+- name: add elk repo to apt sources list
   apt_repository:
-    repo: 'deb https://artifacts.elastic.co/packages/6.x/apt stable main'
-    state: "{{ elk_package_state | default('present') }}"
+    repo: "{{ elk_repo.repo }}"
+    state: "{{ elk_repo.state }}"
+    filename: "{{ elk_repo.filename | default(omit) }}"

elk_metrics_6x/vars/variables.yml

@@ -1,3 +1,8 @@
+# elk apt repo
+elk_repo:
+  repo: 'deb https://artifacts.elastic.co/packages/6.x/apt stable main'
+  state: "{{ elk_package_state | default('present') }}"
+
 # elastic search vars
 elastic_port: 9200
 elastic_hap_port: 9201