Merge branch 'dz-fix-initialize-shards' into 'master'

Fix syntax error in initializers/fill_shards

See merge request gitlab-org/gitlab-ce!22871

config/initializers/fill_shards.rb

-return unless Shard.connected?
-return if Gitlab::Database.read_only?
-
-Shard.populate!
+if Shard.connected? && !Gitlab::Database.read_only?
+  Shard.populate!
+end