Low : nfsserver : add a new parameter for specify the rpc_pipefs mount point

The rpc_pipefs mount point is hard-coded in nfsserver RA (/var/lib/nfs/rpc_pipefs), so can't move rpc_pipefs to any directory.

I refered http://wiki.linux-ha.org/HaNFS to setup of HA NFS servers with nfsserver RA, but could not keep the rpc_pipefs directory local.
I solved that problem by rewriting directly the RA (/var/lib/nfs/rpc_pipefs -> /var/lib/rpc_pipefs).

So , I add a new parameter(rpcpipefs_dir) to nfsserver RA , that specify mount point of the rpc_pipefs.
New parameter's default is /var/lib/nfs/rpc_pipefs, so if new parameter is not specified, RA will work the same way as the previous.

For more infomation , please reference to description in the RA.