Check for a different version of LBaaS agent before stop

This patch fixes the task which tries to stop neutron-lbaas-agent.
When the neutron-lbaas-agent is not installed the task fails.
Added tasks to check if there is a different version of the neutron
LBaaS agent service configured before tring to stop it.

Change-Id: I0ae17303903f8eb7909638f6217fe94a9fd9bb6b
Closes-Bug: #1555256

tasks/neutron_lbaas.yml

@@ -20,16 +20,30 @@
 #
 # These tasks will ensure that the correct agent is the only one running.
 
+- name: Check if LBaaS agent v1 upstart script exists when v2 is in neutron_plugins_base
+  stat: path=/etc/init/neutron-lbaas-agent.conf
+  register: neutron_lbaas_upstart
+  when: neutron_lbaasv2 | bool
+
+- name: Check if LBaaS agent v2 upstart script exists when v1 is in neutron_plugins_base
+  stat: path=/etc/init/neutron-lbaasv2-agent.conf
+  register: neutron_lbaasv2_upstart
+  when: neutron_lbaas | bool
+
 - name: Ensure LBaaS v1 agent is stopped when v2 is in neutron_plugins_base
   service:
     name: neutron-lbaas-agent
     enabled: no
     state: stopped
-  when: neutron_lbaasv2 | bool
+  when:
+    - neutron_lbaasv2 | bool
+    - neutron_lbaas_upstart.stat.exists
 
 - name: Ensure LBaaS v2 agent is stopped when v1 is in neutron_plugins_base
   service:
     name: neutron-lbaasv2-agent
     enabled: no
     state: stopped
-  when: neutron_lbaas | bool
+  when: 
+    - neutron_lbaas | bool
+    - neutron_lbaasv2_upstart.stat.exists