improve AppConfig so values can only be retrieved one way

webhcat server makes its configuration available via AppConfig, which extends a hadoop Configuration.

In HCATALOG-587 there was some confusion about how this class is intended to be used. Consider the following:

    // in AppConfig
    public static final String HADOOP_NAME         = "templeton.hadoop";
    public String clusterHadoop()    { return get(HADOOP_NAME); }

    // inherited from Configuration
    public String get(String name);

Note how there are two ways to get a value from the configuration:

    appConfig.get(AppConfig.HADOOP_NAME);
    appConfig.clusterHadoop();

AppConfig should be updated so there's just one way to access values from the configuration.