Explicitly install postgres-9.6 and fix monit pid-file location for it.

roles/common/tasks/postgres.yml

     state: present
   vars:
     packages:
-    - postgresql
+    - postgresql-9.6
     - python-psycopg2
   tags:
     - dependencies

roles/monitoring/files/etc_monit_conf.d_pgsql

-check process postgres with pidfile /var/run/postgresql/9.4-main.pid
+check process postgres with pidfile /var/run/postgresql/9.6-main.pid
   group database
   start program = "systemctl start postgresql"
   stop program = "systemctl stop postgresql"