Mongo does NOT come with Ambari. And yes, it is a pain in the ass to install. Just trust me though, it DOES work. It might make you wish you'd gone into the Trades instead of IT but it will work.
I'm using Oracle Virtual Box with HDP VirtualBox 2.65. This is an old version however I've found it to be a hella less of a giant memory pig than any newer ones. Feel free to experiment. Especially if you have a better computer than the pile of crap I'm using.
So now just do what I say and don't ask any questions my darlings...
Log into Ambari with your SSH client. Make sure to be in root because you'll be installing stuff:
Then cd into the following directory...
cd /var/lib/ambari-server/resources/stacks/HDP/2.65 (or your version of choice)/services
Now you can grab the MongoDB adapter that this guy so kindly built for us...
git clone https://github.com/nikunjness/mongo-ambari.git
Now restart your Ambari service:
sudo service ambari restart
... in fact you should tattoo the above command on the back of your wrist, because if anything is EVER going wrong with your Ambari service first just try this. It's the fancy equivilent of unplugging your computer then plugging it back in again.