Rename default docker user const

* the name default is a bit confusing now that it conditionally depends on
  configuration

Author: acosta11

app/models/runtime/app_model.rb

@@ -8,7 +8,7 @@ class AppModel < Sequel::Model(:apps)
     include Serializer
     APP_NAME_REGEX = /\A[[:alnum:][:punct:][:print:]]+\Z/
     DEFAULT_CONTAINER_USER = 'vcap'.freeze
-    DEFAULT_DOCKER_CONTAINER_USER = 'root'.freeze
+    DOCKER_CONTAINER_ROOT_USER = 'root'.freeze
 
     many_to_many :routes, join_table: :route_mappings, left_key: :app_guid, left_primary_key: :guid, right_primary_key: :guid, right_key: :route_guid
     one_to_many :route_mappings, class: 'VCAP::CloudController::RouteMappingModel', key: :app_guid, primary_key: :guid

app/models/runtime/droplet_model.rb

@@ -138,7 +138,7 @@ def docker_user
         end
       end
 
-      container_user.presence || (Config.config.get(:allow_process_root_user) ? AppModel::DEFAULT_DOCKER_CONTAINER_USER : AppModel::DEFAULT_CONTAINER_USER)
+      container_user.presence || (Config.config.get(:allow_process_root_user) ? AppModel::DOCKER_CONTAINER_ROOT_USER : AppModel::DEFAULT_CONTAINER_USER)
     end
 
     def staging?

app/models/runtime/process_model.rb

@@ -576,7 +576,7 @@ def permitted_users
     end
 
     def docker_run_action_user
-      actual_droplet&.docker_user.presence || (Config.config.get(:allow_process_root_user) ? AppModel::DEFAULT_DOCKER_CONTAINER_USER : AppModel::DEFAULT_CONTAINER_USER)
+      actual_droplet&.docker_user.presence || (Config.config.get(:allow_process_root_user) ? AppModel::DOCKER_CONTAINER_ROOT_USER : AppModel::DEFAULT_CONTAINER_USER)
     end
 
     def non_unique_process_types