@iMadalin,
Well, in that case: again, you should consider to create an extension.
By the way, RackSpace cannot be compared to a Plesk environment, in no way imaginable.
If you consider to offer existing database SERVERS, you should be aware that you would be EITHER having existing (up and running) database servers OR some scripting to launch new database server (scripts like Chef for multiple servers to launch, or even a multiple of Docker containers on one physical server).
In both methods, you would need a scalable and flexible server infrastructure with a lot of maintenance OR minimal maintenance, in the case of a public cloud (this should suit best).
Whatever you try to do, the scalable/flexible server infrastructure is OUTSIDE the Plesk environment (clustered or non-clustered Plesk instances).
In short, the Plesk environment is only there for the sake of hosting and some custom scripts should be added to "make the connections" to before mentioned server infrastructure.
This is the reason why an extension would be best. Keeps it simple, as far as "simple" applies in your goals.
By the way, I am not certain why you want to create multiple database servers, if the Plesk server is a stand-alone machine (for each instance).
Really, try to focus on the clustering of multiple Plesk instance on multiple servers, each of them running one Plesk instance and multiple local mysql based database servers (with one of the other servers functioning as a mirror/slave of the respective database server).
And, if you know somebody at RackSpace, they can really help you with such a (low cost and HA) setup.
However, note that RackSpace has some issues, when it comes to database hosting (i.e. their cloud infrastructure is not the best in the business, it is not yet mature, when compared to the platforms of Google, Amazon and Microsoft).
Finally, note that you will spend a lot of time and effort to something that (on the one hand) can be easily obtained in a simple cloud environment, but (on the other hand) is not really a good solution in practice, for many reasons (one of them being that you re-invent the wheel).
You can believe me, I am running multiple clustered database servers, they are really a pain-in-the-%&$.
Moreover, there is nothing a mysql server can do and a nosql cannot (or even better, a static (structured) storage augmented with nosql and cache).
My advice: keep it simple, try to add an external mysql server first and TEST connection possibilities (with Plesk) AND TEST scalability of the mysql server (that will fail miserably).
Regards....