Check for pt-osc presence before running

[sauloperez]

There's an edge case concerning create table and adding and index.

When you run a create table migration that includes an index creation from a machine that doesn't have percona-toolkit installed, the table will be created but the index will not, as the former doesn't go through pt-online-schema-change.

We should check it and raise a PerconaMigrator::CommandNotFoundError accordingly, before running a CREATE TABLE.

This also applies to any other DDL statements that don't go through pt-online-schema-change.

[sauloperez]

Moved to departurerb#8