restapi : Missing RSDL descriptor for host.install(image)(#919856)

When installing host using RESTapi, there is no option in the
API to select which iso should be used, unlike the UI. Also
there is on indication which ISO is selected.

Change-Id: I103e75154ce025e82ff7c51f358f527436ab30dc
Bug-Url: https://bugzilla.redhat.com/919856
Signed-off-by: Ravi Nori <rnori@redhat.com>