Fix ClusterBootstrap using wrong fallback port value

[Arkatufus]

Fixes #924

The fix isn't obvious at first.
The failure happened at this filtering method, it is filtering out all contact points from the result:

Akka.Management/src/cluster.bootstrap/Akka.Management.Cluster.Bootstrap/Internal/BootstrapCoordinator.cs

Lines 117 to 136 in 28e6d29

	internal static IImmutableList<ResolvedTarget> SelectHosts(
	Lookup lookup,
	int fallbackPort,
	bool filterOnFallbackPort,
	IImmutableList<ResolvedTarget> contactPoints)
	{
	// if the user has specified a port name in the search, don't do any filtering and assume it
	// is handled in the service discovery mechanism
	if (!string.IsNullOrEmpty(lookup.PortName) || !filterOnFallbackPort)
	return contactPoints;
	return contactPoints.GroupBy(t => t.Host).SelectMany(g =>
	{
	if (g.ToList().Count == 1)
	return g;
	if (g.Any(a => a.Port.HasValue))
	return g.Where(a => a.Port != null && a.Port.Value == fallbackPort);
	return g;
	}).ToImmutableList();
	}

There is nothing wrong with this method by itself, the problem is the fallback port passed into it.

If akka.management.cluster.bootstrap.contact-point.fallback-port was not overriden, the default value is null or undefined. By default, the cluster bootstrap settings will try to fill this information by reading the akka.management.http.port HOCON value.
This by itself is not a problem, if we're using pure HOCON configuration. The AkkaManagementSetup class though, is a different story.

AkkaManagementSetup is being applied directly to the AkkaManagementSettings class, changing the settings class without touching the HOCON configuration. By this time, there are two management HTTP port value, the correct one is held inside AkkaManagement.Settings property, the other is the stale value inside System.Settings.Config.

The bug happened because cluster bootstrap is using the stale value inside the HOCON config, and not the correct one in AkkaManagement.Settings.

[Arkatufus]

Do not depend on HOCON config values of another plugin. The HOCON config is not the single source of truth for configuration anymore, since the final settings value can be influenced by a Setup class.

[Arkatufus]

We read the actual settings value directly from the plugin, not the plugin HOCON config

[Aaronontheweb]

LGTM