Do not allow final without RC in cycle-with-rc

Currently validation considers it valid to propose final versions
on a cycle-with-rc model without RC first:

https://review.opendev.org/#/c/613444/2

Relying on human review to catch that sounds risky. Let's fix it.

Change-Id: Id06794f165f0c852fb62dcacd53058bdc756899e
Task: 27762

openstack_releases/cmds/validate.py

@@ -283,8 +283,12 @@ def validate_pre_release_progression(deliv, context):
         return
 
     releases = deliv.releases
-    if len(releases) < 2:
-        print('this rule only applies to the final release in a series')
+
+    # Milestone-based deliverables cannot directly do a final release
+    if len(releases) == 1:
+        if not releases[-1].is_pre_release_version:
+            context.error('A RC release must be present before final release '
+                          'for deliverables following a milestone-based cycle')
         return
 
     previous_release = releases[-2]

openstack_releases/tests/test_validate.py

@@ -3248,16 +3248,12 @@ class TestValidatePreReleaseProgression(base.BaseTestCase):
         self.assertEqual(0, len(self.ctx.warnings))
         self.assertEqual(0, len(self.ctx.errors))
 
-    def test_no_rc(self):
+    def test_missing_rc(self):
         deliverable_data = yamlutils.loads(textwrap.dedent('''
         ---
         team: Release Management
         release-model: cycle-with-rc
         releases:
-          - version: 1.5.1
-            projects:
-              - repo: openstack/automaton
-                hash: be2885f544637e6ee6139df7dc7bf937925804dd
           - version: 1.5.2
             projects:
               - repo: openstack/automaton
@@ -3275,7 +3271,7 @@ class TestValidatePreReleaseProgression(base.BaseTestCase):
         )
         self.ctx.show_summary()
         self.assertEqual(0, len(self.ctx.warnings))
-        self.assertEqual(0, len(self.ctx.errors))
+        self.assertEqual(1, len(self.ctx.errors))
 
     def test_final_follows_rc(self):
         deliverable_data = yamlutils.loads(textwrap.dedent('''