restapi: Fix cloud-init host name

When sending a request to start a VM using cloud-init the host name
should be included using the "address" element, as follows:

  <cloud_init>
    <host>
      <address>the_host_name</address>
    </host>
  </cloud_init>

But this is incorrectly documented in the RSDL metadata as follows:

  action.vm.initialization.cloud_init.hostname: 'xs:string'

This induces wrong documentation in the Python SDK, which in turn
induces a wrong behaviour of the CLI auto-completion. The right way to
document this in the RSDL is the following:

  action.vm.initialization.cloud_init.host.address: 'xs:string'

Change-Id: I5d3159cf476f881c1fe0e6955b1be44f69f94e58
Bug-Url: https://bugzilla.redhat.com/1080033
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>