Move Cisco VPN RESTapi URI strings to constants

Move Cisco VPN RESTapi URI strings to constants

Cisco VPN RESTapi URI strings are currently spread out
throughout the cisco vpn device driver code. This create
challenge towards consistently using REST resource identifiers
as this code gets enhanced in the future. This change moves
those URIs as string contents towards the top of the device
driver file.

This addresses the review comment received during the
cisco-vpnaas BP implementation.

Change-Id: I17e02cca95b1d14d9218f1a2745780bd14c9f89d
Closes-Bug: #1313801