Iceberg: Drop table should purge the data for V2 tables

1. create external table stored by iceberg in orc format. Convert this to iceberg v2 table format via alter table statements. This should ideally have set "'external.table.purge'='true'" property by default which is missing for V2 tables.
2. insert data into it
3. Drop the table.  This drops the metadata information, but retains actual data. 

Set "'external.table.purge'='true'" as default for iceberg (if it hasn't been set yet).