Would I be considered a Masters degree holder right after my course completion? I am currently a MS student looking forward to complete all of my courses by December 2016. 

Will I be recognized as a Masters degree holder right after December 2016 so that my employers can file for H1B as soon as possible after April 1st of 2017? 

Or do I have to wait till my Graduate Commencement Ceremony to be recognized as a degree holder, which is not till May of 2017?
The date on which you finish courses is not necessarily the date when you earn the degree, nor is the date of the commencement ceremony.

What matters is the date on which the degree is conferred by the university. If you're not sure what that date is, ask the registrar's office.

In some cases if the degree conferral date for students completing their coursework in December/January is after the H1B deadline, the registrar may have some kind of special arrangement (see this example from Stanford), which they'll tell you about if you ask.