GlusterFileSystemForTest -- a modular test class so that different hadoop tests can run against the same GFS implementation 

This pull request refactors our setup code from the original TestGluster class into a separate class.  The new GlusterFileSystemForTest can be used as the basis FileSystem implementation for other tests.  

This feature is necessary for the upcoming hadoop FileSystemBaseContract test implementation , which needs to swap in the FileSystem implementation as a class.