Fix permissions on /etc/nginx/sites-enabled

With the switch to rootwrap, the API service now runs as the gunicorn
user but /etc/nginx/sites-enabled is still owned by root.  This updates
the DIB element to ensure its writable by gunicorn for config rendering.

Also makes a trivial update to releasenotes to remove the UNRELEASED
flag from mitaka.

Change-Id: Ieac128e47a44dd48acd00f68cd8e3a9ca15441ec
Closes-bug: #1558577

diskimage-builder/elements/nginx/element-deps

@@ -1 +1,2 @@
 package-installs
+astara

diskimage-builder/elements/nginx/post-install.d/99-set-nginx-sites-enabled

@@ -14,3 +14,6 @@
 # under the License.
 
 rm -rf /etc/nginx/sites-enabled/default
+
+# astara API appliance runs as gunicorn user and needs to create files here
+chown gunicorn.gunicorn /etc/nginx/sites-enabled

releasenotes/source/mitaka.rst

@@ -1,5 +1,5 @@
 ==========================================================
- Astara Appliance Mitaka Series Release Notes (UNRELEASED)
+ Astara Appliance Mitaka Series Release Notes
 ==========================================================
 
 .. release-notes::