Port deployment variables skeleton code from EE

This commit backports method signatures and related implementation to
the CE, to make CE and EE identical. This does not add any features from
EE, it is only aimed to reduce conflicts between CE and EE in the
future.

app/models/ci/build.rb

@@ -257,7 +257,7 @@ module Ci
         variables.concat(project.predefined_variables)
         variables.concat(pipeline.predefined_variables)
         variables.concat(runner.predefined_variables) if runner
-        variables.concat(project.deployment_variables) if has_environment?
+        variables.concat(project.deployment_variables(environment: environment)) if has_environment?
         variables.concat(yaml_variables)
         variables.concat(user_variables)
         variables.concat(project.group.secret_variables_for(ref, project)) if project.group

app/models/project.rb

@@ -1616,10 +1616,8 @@ class Project < ActiveRecord::Base
     end
   end
 
-  def deployment_variables
-    return [] unless deployment_platform
-
-    deployment_platform.predefined_variables
+  def deployment_variables(environment: nil)
+    deployment_platform(environment: environment)&.predefined_variables || []
   end
 
   def auto_devops_variables