Improve UX of MachineHealthCheckStatus fields

Description

MHC currently has the following columns: MaxUnhealthy, ExpectedMachines, CurrentHealthy. Based on this columns it's a bit hard to grasp what's going on.

The fields have the following semantic (or at least godoc):

• .spec.maxUnhealthy: Any further remediation is only allowed if at most "MaxUnhealthy" machines selected by "selector" are not healthy.
• .status.expectedMachines: total number of machines counted by this machine health check
• .status.currentHealthy: total number of healthy machines counted by this machine health check

A few examples:

• field name: unhealthy vs healthy (would be easier to have either healthy or unhealthy twice, if possible)
• expectedMachines: what does expected mean? according to the godoc it's (probably) the number of machines which match the selector
• currentHealthy: not sure if we need current if it's part of the status (as far as I can see in our other types we're not using the current prefix)

/kind feature