Disallow users from changing Server groups during rebalance

Users can change node-server groups during rebalance.

1. I did this - Rebalance and group change both went on successfully. – this should be blocked
2.I tried to add another node and rebalance, and now rebalance cannot proceed, due to "incorrect tag/node"

Rebalance exited with reason {vbmap_error,
<<"panic: chain refers to tag or node with zero count\n\ngoroutine 1

• Users should be disallowed from getting into such situations.
  adding logs.

Rebal failure details
Rebalance exited with reason

{vbmap_error, <<"panic: chain refers to tag or node with zero count\n\ngoroutine 1 [running]:\nruntime.panic(0x8128340, 0x18653b60)\n\t/usr/lib/go/src/pkg/runtime/panic.c:266 +0xac\nmain.(*selectionCtx).noteChain(0x1862f640, 0x18653b58, 0x1, 0x1)\n\t/home/shaleny/dev/membase/repo30/ns_server/deps/vbmap/vbmap.go:211 +0x23b\nmain.buildVbmap(0x1864cae0, 0x3, 0x3, 0x18645870, 0x3, ...)\n\t/home/shaleny/dev/membase/repo30/ns_server/deps/vbmap/vbmap.go:496 +0x504\nmain.VbmapGenerate(0x1861b4a0, 0x3, 0x1, 0x400, 0x1, ...)\n\t/home/shaleny/dev/membase/repo30/ns_server/deps/vbmap/vbmap.go:626 +0x3e3\nmain.main()\n\t/home/shaleny/dev/membase/repo30/ns_server/deps/vbmap/main.go:346 +0xeef\n">>}