nat: pick outside addr based on local addr

nat: pick outside addr based on local addr

Use outside addresses more evenly by using local address to pick from
pool of addresses. This ensures stability from POV of remote host -
an internal host always gets translated using the same outside address,
so it doesn't appear to be "hopping". Also, this avoids all hosts
being translated using the first address, which helps avoid needless
recaptchas and the like.

Exact assignment depends on internal ordering of addresses - local address
is used to pick an offset into internal vector. If that address cannot be
used, a linear search is performed as a fallback mechanism to find a possible
translation.

Type: improvement
Signed-off-by: Klement Sekera <ksekera@cisco.com>
Change-Id: I7ccb1da1dda5537f5d30d2f4cb48024f4b51c1a4