Install fping package

Now, we can't use os-fping API by default because devstack doesn't
install the fping package.
This commit adds installing the package and enabling to use os-fping
API.

Change-Id: Ibdc7479a9038321e4fc3953774a6f3e1dac90530
Closes-Bug: #1287468
diff --git a/files/apts/n-api b/files/apts/n-api
index e0e5e7f..b4372d9 100644
--- a/files/apts/n-api
+++ b/files/apts/n-api
@@ -1,2 +1,3 @@
 python-dateutil
 msgpack-python
+fping