fix thank you message on order completion

thank you message was no more displayed since it was using a wrong
params to test when order was just completed.
This solves the issue by using flash[:commerce_tracking] param
to test when to display the message. An helper method has been
added to avoid code duplication.

It follows this [mailing list thread](https://groups.google.com/forum/#!searchin/spree-user/Where$20is$20the$20checkout_complete$20key$20added$20to$20params$3F%7Csort:relevance%7Cspell:false/spree-user/GxQtP33m5-8/CtWyO1enJ34J)