What type of PR is this?

/kind bug

What this PR does / why we need it:

The type spec in CAPA says

// SSHKeyName is the name of the ssh key to attach to the instance. Valid values are empty string
// (do not use SSH keys), a valid SSH key name, or omitted (use the default SSH key name)
// +optional
SSHKeyName *string `json:"sshKeyName,omitempty"`

but in launch template creation, CAPA treats "" and nil the same and there's no fallback to a default value. The two must compare equal or else we keep refreshing instances by mistake.

Checklist:

• squashed commits
• includes documentation
• includes emoji in title
• adds unit tests
• adds or updates e2e tests

Release note:

Print reason for launch template needing update, fix constant instance refreshes for AWSMachinePool