gitignore: add _etcd_data directory ignores

After 9a57d1067d8613cd5aa8bd6e00b7522553784770 we want to ignore all of
these _etcd_data directories.

.gitignore

@@ -4,5 +4,5 @@
 /machine*
 /bin
 .vagrant
-*.etcd
+/*_etcd_data
 etcd