Allow setting a cluster offline

[vsliouniaev]

Fixes: #659

Hi! I'm looking for guidance on the general approach here.

Overall this functions fine in my in-cluster testing but I'm only using a small sub-set of functionality.

[morimoto-cybozu]

@vsliouniaev
Hi! Thank you for committing!
Are you wondering if you can move this pull request forward for review?
Your commit seems very reasonable and proper to me. But it seems to lack test code for the new property. How about adding a new It() case to controllers/mysqlcluster_controller_test.go? The case of It("should delete all related resources") would be a useful reference.
Please let us know if you feel you need another type of help.

[vsliouniaev]

Certainly, was looking for some initial feedback on the overall approach

[morimoto-cybozu]

@vsliouniaev
In terms of code modification approach, it seems good to add a new property for an offline cluster. And the motivation for configuring a cluster as "offline" seems reasonable to me.
In terms of review process approach, making a pull request is enough.