[RSGROUP] Master hosting regions messes up rsgroup calculation

In master branch, the Master poses as a RegionServer but it only carries hbase:meta by default. This messes up rsgroup assignment checks and calculations at an extreme when there is only one server in the default group and this server is Master. A move of all servers but this 'fake' RegionServer looks safe because there is a server to pick up all regions shed by the moved servers.

All non-hbase:meta regions are dropped and put into the FAILED_OPEN state.

There are a few things up here.

• Master looks like a RegionServer in much of the system when we are asking for the list of online servers but it is not a full-blown regionserver; the balancer and/or AM are giving out misleading views on the cluster at least as far as rsgroup is concernd.
• rsgroup shouldn't have to do assignment calculations; it should have API to ask the AM to do it for it.

I'm filing this blocker and then putting it aside for now. AM is being redone currently. Will come back when it has been rejiggered. At a minimum, can add check to rsgroup but preferrably can add necessary API to AM for rsgroup to use.