apache: set a default 5-second GracefulShutdownTimeout

The GracefulShutdownTimeout specifies the maximum time servers should continue
to run after receiving a graceful-stop signal and waiting for client
connections to close. The default, as I discovered is 0 -- which means wait
indefinitely!

I strongly suspect that the reason we don't set this is that it was added in
Apache 2.2, whereas our Apache configs predate that.

Change-Id: I4d8fd75025e588fcb3cc486c2c473a3c19748c4e