Expose environments tier over API

Changelog: added

doc/api/environments.md

@@ -36,6 +36,7 @@ Example response:
     "slug": "review-fix-foo-dfjre3",
     "external_url": "https://review-fix-foo-dfjre3.gitlab.example.com",
     "state": "available",
+    "tier": "development",
     "created_at": "2019-05-25T18:55:13.252Z",
     "updated_at": "2019-05-27T18:55:13.252Z",
     "enable_advanced_logs_querying": false,
@@ -147,6 +148,7 @@ Example of response
   "slug": "review-fix-foo-dfjre3",
   "external_url": "https://review-fix-foo-dfjre3.gitlab.example.com",
   "state": "available",
+  "tier": "development",
   "created_at": "2019-05-25T18:55:13.252Z",
   "updated_at": "2019-05-27T18:55:13.252Z",
   "enable_advanced_logs_querying": false,

lib/api/entities/environment.rb

@@ -6,6 +6,7 @@ class Environment < Entities::EnvironmentBasic
       include RequestAwareEntity
       include Gitlab::Utils::StrongMemoize
+      expose :tier
       expose :project, using: Entities::BasicProjectDetails
       expose :last_deployment, using: Entities::Deployment, if: { last_deployment: true }
       expose :state

spec/fixtures/api/schemas/public_api/v4/environment.json

@@ -4,6 +4,7 @@
     "id",
     "name",
     "slug",
+    "tier",
     "external_url",
     "state",
     "created_at",
@@ -13,6 +14,7 @@
     "id": { "type": "integer" },
     "name": { "type": "string" },
     "slug": { "type": "string" },
+    "tier": { "type": "string" },
     "external_url": { "$ref": "../../types/nullable_string.json" },
     "last_deployment": {
       "oneOf": [