devstack needs to run apt-get update

Change-Id: I19724bbbbf6dde38b6414697850816eb9266d68e

devstack.yml

@@ -31,6 +31,10 @@
         password: password
         project: mini-mon
         role: monasca-agent
+  pre_tasks:
+    - name: apt-get update
+      apt: update_cache=yes
+      ignore_errors: true
   tasks:
     - name: Setup the monasca cli credentials in the default environment
       copy: src=tests/env.sh dest=/etc/profile.d/monasca_cli.sh owner=root group=root mode=0644