Enable default SNAT from networks connected to a router indirectly

Make outgoing packets to an external interface SNATed regardless
of source address of the packets. As a result of deep review,
any problem was not found with this change.

Change-Id: I71a1288633bb6af2951d571540bbb9ec5e1270e2
Closes-bug: #1386041
Merge "Enable default SNAT from networks connected to a router indirectly"