Add etc/neutron/*.sample files to be ignored by git

After migration of networking-ovn code to neutron there is
e.g. etc/neutron/ovn.ini.sample file generated now and it should be
ignored by git.

Change-Id: I0d2c703d57552f8fcecc25b07c5e0d8b5aa67c29
Related-Blueprint: neutron-ovn-merge

.gitignore

@@ -45,6 +45,7 @@ nosetests.xml
 
 etc/*.sample
 etc/neutron/plugins/ml2/*.sample
+etc/neutron/*.sample
 
 # Complexity
 output/*.html