[SPARK-16482][SQL] Describe Table Command for Tables Requiring Runtime Inferred Schema

#### What changes were proposed in this pull request?
If we create a table pointing to a parquet/json datasets without specifying the schema, describe table command does not show the schema at all. It only shows `# Schema of this table is inferred at runtime`. In 1.6, describe table does show the schema of such a table.

~~For data source tables, to infer the schema, we need to load the data source tables at runtime. Thus, this PR calls the function `lookupRelation`.~~

For data source tables, we infer the schema before table creation. Thus, this PR set the inferred schema as the table schema when table creation.

#### How was this patch tested?
Added test cases

Author: gatorsmile <gatorsmile@gmail.com>

Closes #14148 from gatorsmile/describeSchema.