Merge branch '24059-post-facto-fixups' into 'master'

Fixups to "Round-robin repository storage"

## What does this MR do?

* Simplifies a method in application_settings.rb
* Correctly marks a migration as needing downtime
* Documents the requirement for renamed columns to be 

## Are there points in the code the reviewer needs to double check?

Should any of these changes be split out? Ideally we'd get this into the same point release as !7273

## Why was this MR needed?

Post-facto review of !7273

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- [X] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [X] API support added
- Tests
  - [X] Added for this feature/bug
  - [x] All builds are passing
- [X] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [X] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [X] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [X] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Related to #24059

/cc @yorickpeterse @rspeicher

See merge request !7287

app/models/application_setting.rb

@@ -202,11 +202,7 @@ class ApplicationSetting < ActiveRecord::Base
   end
 
   def repository_storages
-    value = read_attribute(:repository_storages)
-    value = [value] if value.is_a?(String)
-    value = [] if value.nil?
-
-    value
+    Array(read_attribute(:repository_storages))
   end
 
   # repository_storage is still required in the API. Remove in 9.0

db/migrate/20161103171205_rename_repository_storage_column.rb

@@ -5,12 +5,12 @@ class RenameRepositoryStorageColumn < ActiveRecord::Migration
   include Gitlab::Database::MigrationHelpers
 
   # Set this constant to true if this migration requires downtime.
-  DOWNTIME = false
+  DOWNTIME = true
 
   # When a migration requires downtime you **must** uncomment the following
   # constant and define a short and easy to understand explanation as to why the
   # migration requires downtime.
-  # DOWNTIME_REASON = ''
+  DOWNTIME_REASON = 'Renaming the application_settings.repository_storage column'
 
   # When using the methods "add_concurrent_index" or "add_column_with_default"
   # you must disable the use of transactions as these methods can not run in an

doc/development/what_requires_downtime.md

@@ -66,6 +66,12 @@ producing errors whenever it tries to use the `dummy` column.
 As a result of the above downtime _is_ required when removing a column, even
 when using PostgreSQL.
 
+## Renaming Columns
+
+Renaming columns requires downtime as running GitLab instances will continue
+using the old column name until a new version is deployed. This can result
+in the instance producing errors, which in turn can impact the user experience.
+
 ## Changing Column Constraints
 
 Generally changing column constraints requires checking all rows in the table to