Updates to job submission and launch on YARN clusters

This PR mostly deals with an issue in `YarnConfigurationConstructor` where the configuration wouldn't be searched properly. However, it also contains the following updates:

  * JobJarMaker no longer tries to overwrite files that exist twice on the classpath. Instead, it will print a message into FINEST and ignore the second file with the same name.
  * LaunchClass now provides more elaborate error checking
  * Updated the classpath assembly to allow for a prefix to the classpath to be provided by the runtime. This is helpful to make sure that e.g. the local YARN configuration is always first in the classpath
  * Added new class `HadoopEnvironment` to provide constants for the names of the various Hadoop environment variables.
  * Simplified `YarnConfigurationConstructor` to rely on the classpath to find the configuration
  * `YarnClasspathProvider` now uses the `YarnConfiguration` to assemble the classpath instead of relying on environment variables
  * `HelloReefYarn` now only adds its own JAR file to the Driver classpath, assuming that everything else is already on the Driver. This assumes that we run via `yarn jar reef-examples-?-shaded.jar com.microsoft.reef.examples.hello.HelloReefYarn`
is that the right sequence of calls? we should probably do `f.exists() && f.isFile() && f.canRead()` (although I am not 100% sure - maybe `f.canRead() && f.isFile()` is sufficient)