Perform stack check by loading models without waiting on migrations (#3925)

* Perform stack check by loading models without waiting on migrations

We do this because we want to be able to call the stack checker
prior to running migrations. This is so that if the stack checker fails
the deployment then new migrations will not have run and we are left
a clean state.

Co-authored-by: Sam Gunaratne <[email protected]>

* Checking the stacks table exists to prevent stack_check error on a clean install

Co-authored-by: Sam Gunaratne <[email protected]>

---------

Co-authored-by: Sam Gunaratne <[email protected]>

Author: xandroc

lib/tasks/stack_check.rake

@@ -2,7 +2,10 @@ namespace :stacks do
   desc 'Check Installed Stacks'
   task stack_check: :environment do
     logger = Steno.logger('cc.stack')
-    VCAP::CloudController::DB.load_models(RakeConfig.config.get(:db), logger)
+    db = VCAP::CloudController::DB.connect(RakeConfig.config.get(:db), logger)
+    next unless db.table_exists?(:stacks)
+
+    VCAP::CloudController::DB.load_models_without_migrations_check(RakeConfig.config.get(:db), logger)
     RakeConfig.config.load_db_encryption_key
     require 'models/runtime/buildpack_lifecycle_data_model'
     require 'models/runtime/stack'