[SPARK-5926] [SQL] make DataFrame.explain leverage queryExecution.logical

DataFrame.explain return wrong result when the query is DDL command.

For example, the following two queries should print out the same execution plan, but it not.
sql("create table tb as select * from src where key > 490").explain(true)
sql("explain extended create table tb as select * from src where key > 490")

This is because DataFrame.explain leverage logicalPlan which had been forced executed, we should use  the unexecuted plan queryExecution.logical.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #4707 from yanboliang/spark-5926 and squashes the following commits:

fa6db63 [Yanbo Liang] logicalPlan is not lazy
0e40a1b [Yanbo Liang] make DataFrame.explain leverage queryExecution.logical