*: Optimize cluster condition and state.

[runkecheng]

trafficstars

What type of PR is this?

/enhancement

Which issue(s) this PR fixes?

Fixes #

What this PR does?

Summary:

1. Use separate file(mysqlcluster.go) to save the definition of cluster status.
2. Add conditions related API. keep the use of condition is consistent with K8S.
3. Unit test for mysql status.
4. Judge the status of the cluster according to the change of the template and replicas.
5. New kubectl get columns: Available
6. Simplify the logic of status switching

Special notes for your reviewer?