Update NEWS for the 2.1.5 release

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>

NEWS

 News
 ====
 
+Version 2.1.5
+-------------
+
+*(Unreleased)*
+
+A small bugfix release:
+  - Fix disk adoption: broken by strict --disk option checking in 2.1.4
+  - Fix batch-create: broken in the whole 2.1 series due to a lookup on
+    a non-existing option
+  - Fix instance create: the --force-variant option was ignored
+  - Fix pylint 0.21 compatibility and warnings with python 2.6
+  - Fix modify node storage with non-fqdn arguments
+
 Version 2.1.4
 -------------