Fix index name to Rails default to ensure idempotency

See https://gitlab.com/gitlab-com/infrastructure/issues/3822

db/migrate/20180308052825_add_section_name_id_index_on_ci_build_trace_sections.rb

@@ -3,20 +3,21 @@ class AddSectionNameIdIndexOnCiBuildTraceSections < ActiveRecord::Migration
 
   # Set this constant to true if this migration requires downtime.
   DOWNTIME = false
+  INDEX_NAME = 'index_ci_build_trace_sections_on_section_name_id'
 
   disable_ddl_transaction!
 
   def up
     # MySQL may already have this as a foreign key
-    unless index_exists?(:ci_build_trace_sections, :section_name_id)
-      add_concurrent_index :ci_build_trace_sections, :section_name_id
+    unless index_exists?(:ci_build_trace_sections, :section_name_id, name: INDEX_NAME)
+      add_concurrent_index :ci_build_trace_sections, :section_name_id, name: INDEX_NAME
     end
   end
 
   def down
     # We cannot remove index for MySQL because it's needed for foreign key
     if Gitlab::Database.postgresql?
-      remove_concurrent_index :ci_build_trace_sections, :section_name_id
+      remove_concurrent_index :ci_build_trace_sections, :section_name_id, name: INDEX_NAME
     end
   end
 end