Support generic Hadoop Compatible Filesystem

This is a shortened version of PR #358, with focus on filesystem only.
Problem 1) Configuration.add{Default}Resources doesn't check if core-site.xml exists. Hadoop doesn't care if core-site.xml exists. 
https://github.com/apache/hadoop-common/blob/release-1.0.4/src/core/org/apache/hadoop/conf/Configuration.java#L1164

But here, if core-site.xml doesn't exist, the fallthrough path will be executed.
https://github.com/rootfs/tachyon/blob/wip-hcfs/core/src/main/java/tachyon/UnderFileSystemHdfs.java#L60


Problem 2) 
We support multiple underfs profiles (hdfs, gluster, ceph) in core/pom.xml, and thus potential multiple core-site.xml. We have to precise which one to use for the right under fs.