Adding support for subnet-name

Solution taken from the master change id : Ifcb14a9df2377fc137fcc6955f60f58e8e8f2bc4

Corresponding changes for neutron is here : https://review.opencontrail.org/#/c/650/

Change-Id: I2101304c7e22195df3fb8ac9bb5936b8b6249303
Merge "Adding support for subnet-name" into R1.06