Add metricd workers support in gnocchi

Depending on the environment, gnocchi workers
uses several controller resources RAM/CPU,
this option makes it configurable.

Also, configured to 1 in environments/low-memory-usage.yaml
which will reduce the service footprint in i.e. CI

Change-Id: Ia008b32151f4d8fec586cf89994ac836751b7cce
Closes-bug: #1626473

environments/low-memory-usage.yaml

@@ -9,6 +9,7 @@ parameter_defaults:
   NovaWorkers: 1
   SaharaWorkers: 1
   SwiftWorkers: 1
+  GnocchiMetricdWorkers: 1
 
   ApacheMaxRequestWorkers: 32
   ApacheServerLimit: 32

puppet/services/gnocchi-metricd.yaml

@@ -21,6 +21,10 @@ parameters:
   MonitoringSubscriptionGnocchiMetricd:
     default: 'overcloud-gnocchi-metricd'
     type: string
+  GnocchiMetricdWorkers:
+    default: ''
+    description: Number of workers for Gnocchi MetricD
+    type: string
 
 resources:
   GnocchiServiceBase:
@@ -39,5 +43,6 @@ outputs:
       config_settings:
         map_merge:
           - get_attr: [GnocchiServiceBase, role_data, config_settings]
+          - gnocchi::metricd::workers: {get_param: GnocchiMetricdWorkers}
       step_config: |
         include ::tripleo::profile::base::gnocchi::metricd