nat: nat44-ed pool address allocation improvement

nat: nat44-ed pool address allocation improvement

This patch improves algorithm responsible for choosing nat pool address
during dynamic session creation.

Patch synchronizes nat pool address attributes with vpp interfaces doing
so gives nat option to determine correct nat pool address  when creating
dynamic session.

Improvements:
    * match dst address subnet and src fib with nat pool address
    * for output-feature also fallback match of tx_sw_if_index with nat
      pool address

Type: improvement

Change-Id: I594638bc76fc1153f582add376838b4b72ff573a
Signed-off-by: Filip Varga <fivarga@cisco.com>