vic-machine: registry-ca configure behaviour should replace instead of add

[hickeng]

Story
As a VCH admin I want to be able to update registry CAs when they change.
As a VCH admin I want to be able to revoke registry CA if needed.

Details
The configure behaviour for registry-ca is to add rather than to replace (by report - unconfirmed). This is both counter to the behaviour of other configure options and prevents easy removal of currently configured registry CAs.

bug1991988

[renmaosheng]

let's provide a correct way of updating registry-ca method.

[YanzhaoLi]

The configure behavior is already replace, and supports multiple --registry-ca arguments.
I've tested, and code in validator.go line 670 conf.RegistryCertificateAuthorities = input.RegistryCAs can prove this too.