Local hadoop install support in tachyon startup scripts

Changes:
* Updated tachyon shell scripts to use dynamically generated TACHYON_CLASSPATH
* Added optional HADOOP_HOME to point to a local hadoop installation.
* Generate Tachyon + Hadoop in TACHYON_CLASSPATH if HADOOP_HOME is defined

These changes allow tachyon to share libraries (specifically UnderFileSystem) with the local Hadoop installation without duplicate JARs or conflicts with the packaged hadoop libraries.  Will also allow use of same/shared Hadoop configuration and eventual refactor/removal of any packaged Hadoop dependencies in build.

This simplifies hadoop configuration and library/JAR duplication.  Changes do not impact current behavior, user must uncomment/set the HADOOP_HOME in tachyon-env.sh first.

Somewhat unrelated fix:
* Updated the default JAVA path in linux to the generic symbolic link (will support most JREs without additional config)

Would it not be simpler to use `hadoop classpath` since that exists in both 1.x and 2.x?