[SPARK-15952][SQL] fix "show databases" ordering issue

## What changes were proposed in this pull request?

Two issues I've found for "show databases" command:

1. The returned database name list was not sorted, it only works when "like" was used together; (HIVE will always return a sorted list)

2. When it is used as sql("show databases").show, it will output a table with column named as "result", but for sql("show tables").show, it will output the column name as "tableName", so I think we should be consistent and use "databaseName" at least.

## How was this patch tested?

Updated existing test case to test its ordering as well.

Author: bomeng <bmeng@us.ibm.com>

Closes #13671 from bomeng/SPARK-15952.