Add RAPI client utility module with RAPI PollJob function

The RAPI client module shouldn't depend on any Ganeti module, yet it's
useful to have some Ganeti-specific code, like a PollJob function for
RAPI.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: David Knowles <dknowles@google.com>