Consider supporting multiple --ignition-* switches in coreos-installer install for parity with coreos-installer iso customize

[jlebon]

Feature Request

Desired Feature

coreos-installer iso customize supports passing multiple e.g. --dest-ignition paths and merging them for you. Obviously we want to encourage automation, and that use case is covered but it'd be nice if coreos-installer install supported this as well by allowing multiple --ignition-* switches to be passed and merging the configs for you.

Other Information

This came up as a way to more easily apply machine-specific customizations to a node where the bulk of the config is in a shared config. This is a common issue in OCP-land (openshift/machine-config-operator#1720).