Helm chart: use secrets for credentials

Stores authentication data (MySQL & Artemis) in secrets instead of directly templating it on the manifests.

The MySQL deployment was also refactored to properly create all needed databases and update user passwords without templating the passwords in a SQL file that was then stored in a configmap. Deployments were also updated to use the correct MySQL user instead of the root user.

Closes #51 (closed) Closes #53 (closed)