Simplify tag specification

As suggested by Grzegorz, we can be more declarative in our
configuration of the tags the Entrypoint cares about

Here we add a tags method to the entrypoint class, and use that to
declare the tags an entrypoint is interested in (:core for the existing
Test::Instance, :core and :mattermost for Test::Integration::Mattermost)

We declare everything in specs/features that isn't in
specs/features/mattermost, to have the tag :core, though that clean up
better if we move the integration tests out of specs/features.

qa/qa/scenario/entrypoint.rb

@@ -5,10 +5,17 @@ module QA
     # including staging and on-premises installation.
     #
     class Entrypoint < Template
+      def self.tags(*tags)
+        @tags = tags
+      end
+
+      def self.tag_switches
+        @tags.map { |tag| ['-t', tag.to_s] }
+      end
+
       def perform(address, *files)
         Specs::Config.perform do |specs|
           specs.address = address
-          configure_specs(specs)
         end
 
         ##
@@ -17,13 +24,9 @@ module QA
         Runtime::Release.perform_before_hooks
 
         Specs::Runner.perform do |specs|
-          specs.rspec('--tty', files.any? ? files : 'qa/specs/features')
+          specs.rspec('--tty', self.class.tag_switches, files.any? ? files : 'qa/specs/features')
         end
       end
-
-      protected
-
-      def configure_specs(specs) end
     end
   end
 end

qa/qa/scenario/test/instance.rb

@@ -6,6 +6,7 @@ module QA
       # including staging and on-premises installation.
       #
       class Instance < Entrypoint
+        tags :core
       end
     end
   end

qa/qa/scenario/test/integration/mattermost.rb

@@ -7,11 +7,7 @@ module QA
         # including staging and on-premises installation.
         #
         class Mattermost < Scenario::Entrypoint
-          protected
-
-          def configure_specs(specs)
-            specs.exclusion_filter[:mattermost] = false
-          end
+          tags :core, :mattermost
         end
       end
     end

qa/qa/specs/config.rb

@@ -10,7 +10,6 @@ module QA
   module Specs
     class Config < Scenario::Template
       attr_writer :address
-      attr_accessor :exclusion_filter
 
       def initialize
         @address = ENV['GITLAB_URL']
@@ -34,7 +33,9 @@ module QA
             mocks.verify_partial_doubles = true
           end
 
-          config.exclusion_filter = @exclusion_filter
+          config.define_derived_metadata(file_path: %r{/specs/features/(?!mattermost/)}) do |metadata|
+            metadata[:core] = true
+          end
 
           config.define_derived_metadata(file_path: %r{/specs/features/mattermost/}) do |metadata|
             metadata[:mattermost] = true