Make iallocator work with offline nodes

This patch changes the iallocator framework to work with and properly
export to plugins offline nodes. It does this by only exporting the
static configuration data for those nodes, and not attempting to parse
the runtime data.

The patch also fixes bugs in iallocator related to the RpcResult
conversion, changes the should_run to admin_up attribute name (as per
the internals change), and adds “-I” as a short option for
“--iallocator” in gnt-instance, gnt-backup and burnin.

Reviewed-by: ultrotter