Initial implementation for dynamic SNAT

Initial implementation for dynamic SNAT

allocation pools is needed in an external network
 -> VPP renderer will use external subnet as a pool
    for dynamic SNAT.
     - isTenant = false
     - allocationPool is specified
 -> traffic passing through an interface set to
    'outside NAT' will be automatically translated

Change-Id: I18077d0fa31ae1ae3b2fde13aa12c2d41c3fdecb
Signed-off-by: Tomas Cechvala <tcechval@cisco.com>