Add floating IP support in external switch for the Cisco Nexus plugin

Scope: Adding support to the Cisco plugin for moving floating IP support from the network node to an external Nexus switch.

Use Cases: Neutron with the Cisco plugin.

Implementation Overview: Adding support to the Cisco plugin for the implementation of floating IPs in an external Cisco Nexus switch, rather than in the network node.

Data Model Changes: n/a

Configuration variables: n/a

API's: n/a

Plugin Interface: n/a

Required Plugin support: n/a

Dependencies: n/a

CLI Requirements: n/a

Horizon Requirements: n/a

Usage Example: n/a

Test Cases: n/a