Use single MySQL database in Kubeflow bundle

[i-chvets]

Description

There are two MySQL databases in Kubeflow: kfp-db and katib-db. When MySQL charm was introduced both of those databases were replaced with MySQL charm (instead of out-of-support MariaDB).
MySQL charm consumes a lot of resources which causes slow bring up. In many cases, smaller deployments do not require two databases. In additiom, in development it causes significant slow downs.
To solve this problem it is proposed to have a single MySQL database charm in default Kubeflow deployment with constraints to minimize resource use.
A method should be provided to allow for in-field deployment with two databases.

Design

• Two existing charms kfp-db and katib-db are removed from the bundle along with their relations to kfp-api and katib-db-manager
• A single database MySQL charm kf-mysql-db is added to the bundle.
• Two relations are created: [katib-db-manager:relational-db, kf-mysql-db:database] and [kfp-api:relational-db, kf-mysql-db:database].
• An overlay is created that specifies two databases - kfp-db and katib-db - and their corresponding relations to be used in-field deployments.

Implementation

PR was created, but later closed to allow for more testing: #590
It should be revisited when decision is made when to introduce this fature.