l3_agent: make process_router more robust

l3_agent: make process_router more robust

If internal_network_added/removed fails, _sync_routers_task will call
process_router to do fault recovery. Because the port is already
added/removed to/from ri.internal_ports, internal_network_added or
internal_network_removed will not be called again.

The patch fix this issue by calling ri.internal_ports.append/removed
only if internal_network_added/removed succeed. Without the patch,
the added testcases would fail.

Change-Id: I2d2e004caa670c1624257c1d7ccc900438b42d08
Co-Authored-By: Hirofumi Ichihara <ichihara.hirofumi@lab.ntt.co.jp>
Closes-Bug: #1255871