feat(snmp_exporter):add config.expand-environment-variables flag to argument specs

Author: Luatan

roles/snmp_exporter/meta/argument_specs.yml

@@ -48,3 +48,8 @@ argument_specs:
       snmp_exporter_config_dir:
         description: "Path to directory with snmp_exporter configuration"
         default: "/etc/snmp_exporter"
+      snmp_exporter_expand_environment_variables:
+        description: 
+          - allows passing environment variables into some fields of the configuration file. 
+          - The username, password & priv_password fields in the auths section are supported. 
+        default: false