Support external mysql database for metadata and pipeline

[Jeffwan]

Currently, we use PersistVolume based mysql database for these projects and upgrade and performance becomes a challenges.

One solution is to use external mysql database, we can add overlays and make this as another option for users.

Address kubeflow/kubeflow#4546

[issue-label-bot]

Issue-Label Bot is automatically applying the labels:

Label	Probability
feature	0.98

Please mark this comment with 👍 or 👎 to give our bot feedback!
Links: app homepage, dashboard and code for this bot.

[Jeffwan]

/cc @Jeffwan