No handling when updating with wrong value(disk size) for VNF updation

Release: Stable Newton
Ubuntu 16.04
Environment for NFV

There is no handling when inputting wrong value(e.g disk size), values are reflecting in tacker vnf-show but actually not updated on VNF.
There should be handling that operation should be fail for wrong value input.

Scenario:
1.) Created VNF with disk size:1 GB
| 701b8b15-2859-4113 | fwvnf | OpenWRT with | {"VDU1": | ACTIVE | 613ab165-a420-4362 | {u'vim_name': | |
| -b2e3-a2ca4af6061a | | services | "192.168.x.x"} | | -813d-ec2936422942 | u'nfv_poc'} | |
+--------------------+--------------------+--------------------+--------------------+----------------+--------------------+--------------------+-----------------------+

2.) Updated VNF firstly with 3GB and then again with 7GB

dharmendra@controller:~/logsnall$ tacker vnf-update fwvnf --config-file firewall_update.yaml
Updated vnf: fwvnf

3.) Values reflected in tacker vnf-show and actually disk size not upgraded
properties: {disk_size: 3 GB, mem_size: 512 MB, num_cpus: 1}

These values shouldn't be reflect and updation should be fail for wrong value input