Add support to --force option in SchemaTool CLI Task

The way it's currently coded, schema-tool task is enabled --force by default.

This means that it'll attempt to execute every single SQL command without actually use error checking in SQL statements. One good example is if you attempt to --drop same schema twice. It'll generate a PDOException to you noticing that table does not exist (on second execution).

Instead of get errors on second attempt, it should gently notify it worked smoothly, except if --force is requested. To support this approach (specifically for --drop, but it should be applied for other types too), it is required to change the way we currently drop tables. Instead of do this:

DROP TABLE users;

It should try this approach:

DROP TABLE IF EXISTS users;

The idea is to --drop --force to execute like it is currently and --drop is only execute if exists approach. Same for --create, which means --force will attempt without care of existance in database. --create without --force check for previous existance and drop/create it again.

Problem is that not all drivers support it natively. Here is what my research gave me so far:

*SQLite* (3.3+)

DROP TABLE IF EXISTS users;

*MySQL* (3.22+)

DROP TABLE IF EXISTS users;

*PostgreSQL*

DROP TABLE IF EXISTS users;

*MS SQL Server*

IF EXISTS(SELECT TABLE_NAME FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME = 'users') DROP TABLE users;

*Oracle*

declare 
  p_exists number :=0;
begin
  select nvl((select 1 from user_tables where table_name='TABLE1'  and rownum=1),0) into p_exists from dual ;
if p_exists = 1 then
  execute immediate '...

This is quite complex... maybe we should try:

IF (SELECT COUNT(1) FROM USER_TABLES WHERE TABLE_NAME = 'users') THEN DROP TABLE users;

Or...

if ((SELECT COUNT(1) FROM USER_TABLES WHERE TABLE_NAME = 'users') = 1) then execute immediate 'drop table users'; end if;

*DB2*

IF EXISTS (SELECT name FROM sysibm.systables WHERE name = 'users' ADN type = 'T') THEN
DROP TABLE users;
END IF;

*Informix*

Only information I got so far. Check if the table exists:

SELECT tabname FROM systables WHERE tabname='users';