Adds delete of extra-dhcp-opt to the client

Add support to delete existing extra-dhcp-opt(s) for a port
via the port-update operation. To use it, set the opt_value=null
of the opt_name that you want deleted (i.e. neutron port-update
--extra-dhcp-opt opt_name=tftp-server,opt_value=null), which will
delete this DHCP option from the port.

Change-Id: I9c2c2474b2bfe8c8f8c3c3cc1513d2b999643d14
Closes-Bug: 1228008
Merge "Adds delete of extra-dhcp-opt to the client"