Fixes #11032 - Replication fields broken in custom validation

[kkthxbye-code]

Fixes: #11032

The issue here is that validation is triggered on the base instance when the form uses replication_fields. So if the user creates a vminterface with name test[1-2] validation is triggered for:

VMInterface with no name
VMInterface with name test1
VMInterface with name test2

For the normal validation this works okay, but for custom validation it results in the validator being triggered for the first VMInterface which is never actually saved. Not sure if there is a smarter way to fix this, my approach is to tag the instance that is never going to be saved with a private variable and make sure we don't run custom validators for these instances.

[jeremystretch]

To really resolve this, we'll need to overhaul the manner in which component replication forms are built and processed; I've opened #11707 to address this. For now, this seems like a reasonable stop-gap. Thanks!