Reserve memory for host

By default cloudstack reserves 1Gb of RAM in hosts
using _dom0_memory field. Add a global setting
"host.reserved.mem.mb" which can used to either
increase or decrese the amount of memory which can be reserved

engine/orchestration/src/main/java/com/cloud/agent/manager/AgentManagerImpl.java

@@ -1979,6 +1979,39 @@ public void updateCapacityOfHosts() {
         }
     }
 
+    @Override
+    public void updateCapacityOfHosts() {
+        Map<Long, List<Long>> hostsByZone = new HashMap<>();
+        boolean allHostMemoryValuesAreValid = true;
+
+        List<HostVO> allHosts = _resourceMgr.listAllHostsInAllZonesByType(Host.Type.Routing);
+        if (CollectionUtils.isEmpty(allHosts)) {
+            return;
+        }
+
+        for (HostVO host : allHosts) {
+
+            boolean updateWasSuccessFull = updateHostMemory(host);
+
+            if (! updateWasSuccessFull){
+                allHostMemoryValuesAreValid = false;
+                continue;
+            }
+
+            Long zoneId = host.getDataCenterId();
+            List<Long> hostIds = hostsByZone.getOrDefault(zoneId, new ArrayList<>());
+            hostIds.add(host.getId());
+            hostsByZone.put(zoneId, hostIds);
+        }
+
+        if (allHostMemoryValuesAreValid) {
+            sendCommandToAgents(hostsByZone,
+                    hostId -> Collections.singletonMap(
+                            ConfigurationManagerImpl.HOST_RESERVED_MEM_MB.key(),
+                            ConfigurationManagerImpl.HOST_RESERVED_MEM_MB.valueIn(_hostDao.findById(hostId).getClusterId()).toString()));
+        }
+    }
+
     private GlobalLock getHostJoinLock(Long hostId) {
         return GlobalLock.getInternLock(String.format("%s-%s", "Host-Join", hostId));
     }