[SPARK-7601] [SQL] Support Insert into JDBC Datasource

Supported InsertableRelation for JDBC Datasource JDBCRelation.
Example usage:
sqlContext.sql(
      s"""
        |CREATE TEMPORARY TABLE testram1
        |USING org.apache.spark.sql.jdbc
        |OPTIONS (url '$url', dbtable 'testram1', user 'xx', password 'xx', driver 'com.h2.Driver')
      """.stripMargin.replaceAll("\n", " "))

sqlContext.sql("insert into table testram1 select * from testsrc")
sqlContext.sql("insert overwrite table testram1 select * from testsrc")

Author: Venkata Ramana Gollamudi <ramana.gollamudi@huawei.com>

Closes #6121 from gvramana/JDBCDatasource_insert and squashes the following commits:

f3fb5f1 [Venkata Ramana Gollamudi] Support for JDBC Datasource InsertableRelation