Fix pattern to exclude generated config file

The current pattern does not match the target path so git still picks
up the file as a change which can be included.

Change-Id: I8a4c0781ecba4c7b2686213623bb312c523b41ac

Author: kajinamit

.gitignore

@@ -43,7 +43,7 @@ nosetests.xml
 # Translations
 *.mo
 
-etc/*.sample
+etc/octavia/conf.d/ovn.conf.sample
 
 # Complexity
 output/*.html