Allow external_stats_reader role in operator

Currently the Operator does not handle the role external_stats_reader

1. * spec.roles.name: Unsupported value: "external_stats_reader": supported values: "admin", "cluster_admin", "security_admin", "ro_admin", "replication_admin", "query_external_access", "query_system_catalog", "analytics_reader", "bucket_admin", "views_admin", "fts_admin", "bucket_full_access", "data_reader", "data_writer", "data_dcp_reader", "data_backup", "data_monitoring", "replication_target", "analytics_manager", "views_reader", "fts_searcher", "query_select", "query_update", "query_insert", "query_delete", "query_manage_index", "mobile_sync_gateway"

We should add this since Couchbase server has this role.