TACHYON-193 TACHYON-194 First drop of Tachyon neo configuration

The PR includes:
-) TACHYON-193
Add new Tachyon properties file configuration to support file based configuration.

Add new code to support Tachyon properties configuration for runtime. 
This will be file based could be overriden via site specific configuration file or override collection in the TachyonConf class.
-) TACHYON-194
Add TachyonConf class to unify all Tachyon configurations.
Use single class TachyonConf to manage the configurations properties as key-value store.
For this particular constants: DEFAULT_HADOOP_UFS_PREFIX I agree we need to remove it. It was just used for testing.

As for others, static constants are just use for convenience and reduce typos.
For example: 
TachyonConf tachyonConf = new TachyonConf();
tachyonConf.get(Constants.TACHYON_HOME, null);

The use of Constants.TACHYON_HOME is to prevent typo of typing "tachyon.home" when used in the code.

Note that the actual value of the properties is not set via the static variables.