(PUP-744) Allow keepalive timeout to be configurable

Previously the http keepalive timeout was hard coded to be 15 seconds.
This commit adds a new puppet setting, http_keepalive_timeout, which
defaults to 4 seconds. The reason for the smaller value, is because
Apache 2.2 and 2.4 have a corresponding server side keepalive timeout,
which defaults to 5 seconds.

If the client timeout were greater than or equal to the server timeout,
then the client may try to reuse a connection from the pool that the
server has almost certainly closed.