[Tracking Upstream] Bump HAProxy to 2.2.15

As a cluster administrator, 

I want OpenShift to include a recent HAProxy version, 

so that I have the latest available performance and security fixes.  

We should strive to follow upstream HAProxy releases by bumping the HAProxy version that we ship in OpenShift with every 4.y release, so that OpenShift benefits from upstream performance and security fixes, and so that we avoid large version-number jumps when an urgent fix necessitates bumping to the latest HAProxy release.  This bump should happen as early as possible in the OpenShift release cycle, so as to maximize soak time.  

For OpenShift 4.9, this means bumping from HAProxy 2.2.9 to 2.2.13, or possibly a later 2.2.z release should one ship before we do the bump.  

We may consider bumping HAProxy again during the OpenShift 4.9 release cycle if upstream ships additional 2.2.z releases during the 4.9 development cycle.  However, we will need to weigh the risks and available remaining soak time in the release schedule before doing so, should that contingency arise.