freebsd: try using wait_to_add_keys

A problem was found that with some cloud systems
packer was sending a request to set ssh key information
before the server was ready to accept an update.

Let's try waiting a bit before attempting to generate
host keys on a pristine instance.

Signed-off-by: Kelley Spoon <kelley.spoon@linaro.org>
Change-Id: Ie0517e671a125ff1072482323fd7d2715cfa6e1d
1 file changed