https://cwiki.apache.org/confluence/display/Hive/LanguageManual+VariableSubstitution

This is a block issue for the CLI user, it impacts the existed hql scripts from Hive.

Author: Cheng Hao <hao.cheng@intel.com>

Closes #4003 from chenghao-intel/substitution and squashes the following commits:

bb41fd6 [Cheng Hao] revert the removed the implicit conversion
af7c31a [Cheng Hao] add hql variable substitution support