S3: Remove DFS and single-filesystem assumptions from HdfsTable

The FE and catalog code makes assumptions that:
1) All files are on the default FS.
2) The filesystem type is DistributedFileSystem.

Remove those two assumptions from HdfsTable.  Following this change,
HdfsTable is a bit of a misnomer since this metadata can be used for
file-based tables on any Hadoop filesystems.  A partitioned table can
even span multiple filesystems.  However, let's defer any renaming until
later (if at all -- file accesses will still be through libHDFS in all
cases).

The motivation is to support S3, but really this is a necessary step to
support any other Hadoop filesystems besides HDFS in Impala.  Also note
this change is necessary but not sufficient for querying from S3.

For now, this change is only regression tested to ensure it doesn't
break existing functionality.  Once a few more changes are in we can
start testing against S3 and potentially other filesystems (e.g. local).

Change-Id: Iddd08067296ef65cd663c77493881407c3afc5a2
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5112
Reviewed-by: Daniel Hecht <dhecht@cloudera.com>
Tested-by: jenkins