Make MiniAccumuloCluster default to using RawLocalFileSystem when not using MiniDFSCluster

RawLocalFileSystem does not do checksumming and sync actually works which makes it a much more reliable testing FileSystem over LocalFileSystem (in which sync is not guaranteed to work as we want it).

We should default to using RawLocalFileSystem (configured via "fs.file.impl" in core-site.xml) and let the user override it if they so choose. This would alleviate having to manually set it in new unit tests/ITs

(originally reported at [ACCUMULO-2478](https://issues.apache.org/jira/browse/ACCUMULO-2478))