Adds info message when using percona migrator

[trecenti]

As a ActiveRecord user
I want to see when PerconaMigrator is being used
So that I know my migrations will use PerconaMigrator

I've added this simple logging every time the logger is created to notify users when Percona Migrator is being used instead of the default one.

[sauloperez]

Thanks for contributing @trecenti ! I like it but I think it's better if you use the particular instance of logger after its instantiated: https://github.com/redbooth/percona_migrator/blob/8280285382f358768df9636fdb6673f2681997a0/lib/active_record/connection_adapters/percona_adapter.rb#L15

I guess you want to notify about the use of the migrator as early as possible. In that case, logging the message before instantiating the PerconaMigratorAdapter seems a good place.

[trecenti]

I guess you want to notify about the use of the migrator as early as possible. In that case, logging the message before instantiating the PerconaMigratorAdapter seems a good place.

That sounds good, I will do this changes.