Always create WAL directory for region server

After HBASE-20708, when master restart, we will scan the wal directory to find out the live servers. In most cases this is OK, as when we create a HRegion instance at RS side, we will create a WAL for it, and the directory which contains the server name will be there, even if user always use SKIP_WAL.

But there could still be problem as the directory is created in the implementation of WAL, not in the initialization of region server, so if user uses DisabledWALProvider then we will be in trouble.

So let's fix it.