Issue #2150407 by chx, bdone, eliza411, andypost: Migrate D6 roles and permissions.

.../lib/Drupal/migrate/Plugin/migrate/destination/Entity.php

.../migrate_drupal/config/migrate.migration.d6_user_role.yml

...grate_drupal/Plugin/migrate/process/d6/NodeUpdate7008.php

...ate_drupal/Plugin/migrate/process/d6/SystemUpdate7000.php

...b/Drupal/migrate_drupal/Plugin/migrate/source/d6/Role.php

.../lib/Drupal/migrate_drupal/Tests/Dump/Drupal6UserRole.php

...ib/Drupal/migrate_drupal/Tests/d6/MigrateUserRoleTest.php

.../Drupal/migrate_drupal/Tests/source/d6/RoleSourceTest.php