[Falcon] Dhcp Bug Fix

- buildReply method refers the leasetime stored in ipAssignment

Change-Id: If0477861696bd76b3c1c14983f013968b4b8e8eb

@@ -354,7 +354,8 @@ public class DhcpManager implements DhcpService {
                 option = new DHCPOption();
                 option.setCode(DHCP.DHCPOptionCode.OptionCode_LeaseTime.getValue());
                 option.setLength((byte) 4);
-                option.setData(ByteBuffer.allocate(4).putInt(leaseTime).array());
+                option.setData(ByteBuffer.allocate(4)
+                        .putInt(ipAssignment == null ? leaseTime : ipAssignment.leasePeriod()).array());
                 optionList.add(option);
 
                 // IP Address Renewal Time.

@@ -208,7 +208,7 @@ public class DistributedDhcpStore implements DhcpStore {
                                     assignment = IpAssignment.builder()
                                             .ipAddress(ipAddr)
                                             .timestamp(new Date())
-                                            .leasePeriod(leaseTime)
+                                            .leasePeriod(existingAssignment.leasePeriod())
                                             .rangeNotEnforced(true)
                                             .assignmentStatus(IpAssignment.AssignmentStatus.Option_RangeNotEnforced)
                                             .subnetMask(existingAssignment.subnetMask())