### Description

Implement health indicator for shard allocation.
This indicator should have same or similar behavior to existing health endpoint.

Aspects to be considered:
* nodes scheduled for reboot should not cause cluster to go yellow (but could cause it go red)
* huge number of unallocated shards should not cause response bloat.
* consider showing allocation explain if possible

Rel: TBD
server/src/main/java/org/elasticsearch/node/Node.java
nodeEnvironment.nodeDataPaths()
                );
                assert nodeMetadata != null;
                assert nodeMetadata.nodeVersion().equals(Version.CURRENT);
                assert nodeMetadata.nodeId().equals(localNodeFactory.getNode().getId());
            } catch (IOException e) {
                assert false : e;
            }
        }
        
        
        final Metadata onDiskMetadata = gatewayMetaState.getPersistedState().getLastAcceptedState().metadata();
        assert onDiskMetadata != null : "metadata is null but shouldn't"; 
        validateNodeBeforeAcceptingRequests(
            new BootstrapContext(environment, onDiskMetadata),
            transportService.boundAddress(),
            pluginsService.filterPlugins(Plugin.class).stream().flatMap(p -> p.getBootstrapChecks().stream()).collect(Collectors.toList())
        );
        clusterService.addStateApplier(transportService.getTaskManager());
        
        coordinator.start(); 
        clusterService.start();
        assert clusterService.localNode().equals(localNodeFactory.getNode())
            : "clusterService has a different local node than the factory provided";
        transportService.acceptIncomingRequests();
        coordinator.startInitialJoin();
        final TimeValue initialStateTimeout = INITIAL_STATE_TIMEOUT_SETTING.get(settings());
        configureNodeAndClusterIdStateListener(clusterService);
        if (initialStateTimeout.millis() > 0) {
            final ThreadPool thread = injector.getInstance(ThreadPool.class);