Clarify what overcloud is controlling in devtest.

KVM was specified as what overcloud controls, but this can actually
be a variety of virtualization options, including QEMU (as used in
testing) and Xen.

This hopefully clarifies that the documentation refers to what the
overcloud controls, not what it is.

Change-Id: Ib83a135f1a2376207ad5aafe96ae33584f52ae8d