exit if pt-online-schema-change is not found

[lefred]

If pt-online-schema-change is not found, we should stop (until we specify we won't use it, if this new feature
gets implemented).

We avoid this and let bad status:

ERROR : Problem while running :
    pt-online-schema-change h=127.0.0.1,P=3306,u=root,p=,D="world",t=City --alter="add PolutionLevel int;" 
    --execute >>online_migration.log 2>&1

[lefred]

check if pt-online-schema-change exists and if not, or stop or use normal alter statement

this is to decide what will be the default and maybe override it in the cfg file with:

alter_allowed=1