Question: Do we need to make changes for Salesforce Fast deploy to deploy our code via ANT migration tool ? OR does Salesforce take care of it automatically ?

I would like to find some documentation which states that we don't have to make any changes for fast deploy.

Answer: 
If you will go through [this](http://releasenotes.docs.salesforce.com/en-us/winter15/release-notes/rn_quick_deployment.htm) article, it is clearly written :

When deploying to non-production environments (sandbox), Apex tests aren’t required and aren’t run automatically. ***When using Metadata API (including the Force.com Migration Tool), Quick Deploy is supported in sandbox only for validations that explicitly enable the execution of tests (for example, via the runAllTests parameter for the Migration Tool)***. For change sets, Quick Deploy is not supported in sandbox because there is no option to enable test runs for change sets.

So it seems like, using Metadata API(ANT), it is supported only in sandbox(with runAllTests paramater enabled)
I may be wrong here. But thought its worth to put my thoughts.

Once you validate using ANT, you can go to salesforce and use the quick deploy feature. Guess as of now, through ANT quick deploy is not supported into prod instance.