Fixing onos-user-key to use ssh-keygen optionally.

Change-Id: I18e20fca4ee403e72fe008d4da00f0b6d5ca7e6e
diff --git a/tools/package/bin/onos-user-key b/tools/package/bin/onos-user-key
index ec496f6..a9924ce 100755
--- a/tools/package/bin/onos-user-key
+++ b/tools/package/bin/onos-user-key
@@ -28,4 +28,4 @@
 (umask 077; touch "$HOME/.ssh/known_hosts")
 
 # Also, remove any previous known keys for the localhost.
-ssh-keygen -f "$HOME/.ssh/known_hosts" -R [localhost]:8101
+which ssh-keygen >/dev/null && ssh-keygen -f "$HOME/.ssh/known_hosts" -R [localhost]:8101