Revert "Disable ironic_prometheus_exporter (#2557)" (#2560)

Disabling ironic prometheus exporter will happen as a global default in
osism. It is not required here.

This reverts commit cd26c5a.

Signed-off-by: Jan Horstmann <horstmann@osism.tech>

Author: janhorstmann

environments/kolla/configuration.yml

@@ -72,7 +72,6 @@ ironic_dnsmasq_dhcp_range: "192.168.112.50,192.168.112.60"
 ironic_dnsmasq_dhcp_ranges:
   - range: "192.168.112.50,192.168.112.60"
 ironic_cleaning_network: "public"
-enable_ironic_prometheus_exporter: false
 
 # ceilometer
 enable_ceilometer_prometheus_pushgateway: "yes"