Fix bootstrap prometheus container location

This change to fix the case that
I won't use prometheus-mysqld-exporter

Change-Id: I1936bbae0172f4e65605d71066dced837bc30f7a

ansible/roles/prometheus/tasks/bootstrap.yml

@@ -13,7 +13,7 @@
       priv: "*.*:PROCESS,REPLICATION CLIENT,SELECT"
       append_privs: "yes"
   run_once: True
-  delegate_to: "{{ groups['prometheus-mysqld-exporter'][0] }}"
+  delegate_to: "{{ groups['prometheus'][0] }}"
   when:
     - enable_prometheus_mysqld_exporter | bool
     - not use_preconfigured_databases | bool