[SPARK-15253][SQL] Support old table schema config key...

[SPARK-15253][SQL] Support old table schema config key "spark.sql.sources.schema" for DESCRIBE TABLE

## What changes were proposed in this pull request?

"DESCRIBE table" is broken when table schema is stored at key "spark.sql.sources.schema".

Originally, we used spark.sql.sources.schema to store the schema of a data source table.
After SPARK-6024, we removed this flag. Although we are not using spark.sql.sources.schema any more, we need to still support it.

## How was this patch tested?

Unit test.

When using spark2.0 to load a table generated by spark 1.2.
Before change:
`DESCRIBE table` => Schema of this table is inferred at runtime,,

After change:
`DESCRIBE table` => correct output.

Author: Sean Zhong <seanzhong@databricks.com>

Closes #13073 from clockfly/spark-15253.