Persistent nfs affinities support for device.

We have site1, site2 deployed with 2 same rootfs. To make all devices be fully used, we wish the devices in 2 sites be treated as a global pool. Another word, one job could be either executed in site1 or site2 as soon as the device is idle.

For some reasons, our job is configured with persistent_nfs, but it can just specify either site1 nfs or site2 nfs. What we really need is: the dispatched device should find the nearest nfs server based on affinities.

This patch to binding a external nfs ip to device, so the device possible to find the nfs server with the same job define, won't care itself been dispatched to which site.