Automatically configure Afterburn provider on platforms that need it

[bgilbert]

CLCT supports setting the Afterburn provider on platforms where that needs to be explicitly configured (OpenStack and CloudStack). If we want to continue requiring that bit of configuration, FCCT should learn to generate it automatically from a platform name specified on the command line. That does mean that FCCT output becomes platform-specific, which isn't ideal.

[bgilbert]

At this point, there's a pretty entrenched expectation that Butane output is platform-independent. We've already switched to a unified Afterburn OpenStack provider in coreos/afterburn#462, and coreos/afterburn#856 tracks doing the same for CloudStack (which is a platform currently not supported by FCOS or RHCOS), so there shouldn't be a need for this.