NSX|V add dhcp-mtu extension to subnet

[OpenStack Infra]

https://review.openstack.org/346503
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.
Your project "openstack/vmware-nsx" is set up so that we directly report the documentation bugs against it. If this needs changing, the docimpact-group option needs to be added for the project. You can ask the OpenStack infra team (#openstack-infra on freenode) for help if you need to.

commit d4fa95168c2ef3cb6ffc8a449c2a3dc89946866b
Author: Adit Sarfaty <email address hidden>
Date: Sun Jul 24 11:50:01 2016 +0300

    NSX|V add dhcp-mtu extension to subnet

    Add subnet extension dhcp-mtu and configure it in option26 of the dhcp binding.
    Also add this column to the nsxv_subnet_ext_attributes DB table.
    This option will be available only from NSX version 6.2.3

    DocImpact: Added dhcp-mtu extension to subnets

    Change-Id: Id2a74a3c089beb61fde6b7c0fd02b207e444c3b7