Cleanup stale bind mounts for port mapping isolator during slave recovery.

Leaked bind mount under /var/run/netns for port mapping isolator is a known issue. There are many ways it can get leaked. For example, if the slave crashes after creating the bind mount but before creating the veth, the bind mount will be leaked. Also, if the detached unmount does not finish in time and the subsequent os::rm fails, the bind mount will be leaked as well.

Since leaked bind mount is inevitable, we need to clean them up during startup (slave recovery).