Fix KeyError during sync_routers

Method sync_routers is used by the L3 agent to query
routers it knows about. Routers and GW ports lists
are populated in two different times, which means that
they can be interleaved by a delete request which
results in gateway ports being missing in one of the
two data structures.

This patch takes care of the race condition.

Closes-bug: #1355409

Change-Id: Id3a6fe145058f690e107bfe7023980ede61cff90
Merge "Fix KeyError during sync_routers"