fix(osmap.yaml): fix PGDG repo file creation for Amazon Linux

postgres/osmap.yaml

@@ -9,3 +9,7 @@ Fedora:
     baseurl: 'https://download.postgresql.org/pub/repos/yum/{{ repo.version }}/fedora/fedora-$releasever-$basearch'
   remove:
     releases: ['9.4', '9.5', '9.6', '10']
+
+Amazon:
+  pkg_repo:
+    baseurl: 'https://download.postgresql.org/pub/repos/yum/{{ repo.version }}/redhat/rhel-7-$basearch'