[Conf] Make default_storage_medium configurable (#2980)

Doris support choose medium when create table, and the cluster balance strategy is dependent
between different storage medium, and most use will not specify the storage medium when create table,
even they kown that they should choose a storage medium, they have no idea about the
cluster's storage medium, so, I think we should make storage_medium and storage_cooldown_time
configurable, and this should be the admin's responsibility.

For Example, if the cluster's storage medium is HDD, but we need to change part of machines to SSD,
if we change the machine, the tablets before change is stored in HDD and they can't find a dest path
to migrate, and user will create table as usual, it will make all tablets stored in old machines and
the new machines will only store a little tablets. Without this config the only way is admin need
to traverse all partitions in cluster and change the property of storage_medium, it will increase
operational and maintenance costs.

So I add a FE config default_storage_medium, so that user can set the default storage medium.

docs/documentation/cn/sql-reference/sql-statements/Data Definition/CREATE TABLE.md

@@ -223,9 +223,9 @@ under the License.
            )
     ```
 
-       storage_medium：        用于指定该分区的初始存储介质，可选择 SSD 或 HDD。默认为 HDD。
+       storage_medium：        用于指定该分区的初始存储介质，可选择 SSD 或 HDD。默认初始存储介质可通过fe的配置文件 `fe.conf` 中指定 `default_storage_medium=xxx`，如果没有指定，则默认为 HDD。
            storage_cooldown_time： 当设置存储介质为 SSD 时，指定该分区在 SSD 上的存储到期时间。
-                                   默认存放 7 天。
+                                   默认存放 30 天。
                                    格式为："yyyy-MM-dd HH:mm:ss"
            replication_num:        指定分区的副本数。默认为 3