platforms, tmt: Rename and restructure cloud variants

We now have more than one cloud system that is essentially a KVM
environment that uses cloud-init to boot, so now we declare that
"generic" and have the OpenStack and Oracle images reuse those
definitions.

Also, in preparation for layered cloud image variants, rename
them from "Cloud-" to "Cloud-Base-". Vagrant variants are
similarly prefixed to make it clear who provides them.

Finally, restructure the layout of the tmt plans to match the
description structure.

tmt/plans/vagrant/base/build-libvirt.fmf

@@ -0,0 +1,8 @@
+summary: Build base Vagrant image for libvirt
+discover:
+    how: fmf
+environment:
+    image_type: oem
+    image_profile: Cloud-Base-Vagrant-libvirt
+execute:
+    how: tmt

tmt/plans/vagrant/base/build-virtualbox.fmf

@@ -0,0 +1,11 @@
+summary: Build base Vagrant image for VirtualBox
+discover:
+    how: fmf
+adjust:
+    enabled: false
+    when: arch != x86_64
+environment:
+    image_type: oem
+    image_profile: Cloud-Base-Vagrant-VirtualBox
+execute:
+    how: tmt