Better way to ensure keepalived before haproxy.

Better way to ensure keepalived before haproxy.

The lastest patchset of https://review.openstack.org/393361 was actually
not working.

The `if defined` idiom depends on *evaluation* order.

At the time it's red in the haproxy.pp class, the line that loads the
class 'haproxy' has still not yet been reached and thus the `defined`
result is false. The constraint is not added.

For this reason, the use of `defined` in module is not advised by
puppetlabs[1].

[1] https://docs.puppet.com/puppet/latest/reference/function.html#defined

Change-Id: Ibd352cb313f8863d62db8987419378bed5b87256
Relates-To: #1638029